THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


ELEMENTS   OF   PLANT   BIOLOGY 


ELEMENTS  OF  PLANT 
BIOLOGY 


A.    G.    TANSLEY,    M.A.,  F.R.S. 

University  Lecturer  in  Botany  and  Examiner  in  Elementary 
Biology  to  the  University  of  Cambridge 


LONDON:  GEORGE  ALLEN  &  UNWIN  LTD. 
RUSKIN  HOUSE,  40  MUSEUM  STREET,  W.C.  i 
NEW  YORK  :  DODD,  MEAD  &  COMPANY 


(All  rights  resen-ed) 
Prittted  in  Great  Brit** 


7/1 
Tl 
WW. 


PREFACE 


THIS  book  is  intended  primarily  for  medical  students 
and  others  who  do  not  necessarily  intend  to  continue 
the  study  of  botany,  but  who  desire  or  are  obliged  to 
obtain  some  elementary  knowledge  of  plants,  particularly 
in  relation  to  general  biology.  It  may  perhaps  also 
be  useful  in  the  highest  forms  of  schools  where  biology 
is  taught,  as  well  as  in  training  colleges.  But  it 
is  unsuitable  for  pupils  under  seventeen. 

The  book  is  based  on  the  first  portion,  which  deals 

mainly    with    plants,    of    the    course    in    Elementary 

Biology  for  the  Preliminary  Examination  in  Science 

and   the   First   Examination  for  the  M.B.   degree  at 

Cambridge.     This  part  of  the  course  has  been  framed, 

in  complete  freedom  as  to  the  material  chosen  and 

its  treatment,  but  in  co-operation  with  the  Professor 

of  Zoology,   to  serve  as  an  introduction  to  Biology 

suitable  for  freshmen,  many  of  whom  know  nothing 

&   whatever   of  the   subject.     This   first   portion   of  the 

^    course,  which  is  covered  by  the  present  work,  occupies 

one    term,   and    comprises    twenty-four    lectures    and 

>   forty-eight  hours  of  practical  work  in  the  laboratory. 

£  The    rest    of    the    course,    which    deals    mainly    with 

p    animals,     comprises     about     thirty-six    lectures     and 

seventy-two    hours    of   practical    work,    and   includes 

some  elementary  treatment  of  heredity  and  evolution. 

No  claim  is  made  to  ideal  pedagogic  method.     The 

problem  we  are  faced  with  at  Cambridge  is  a  strictly 


1028494 


ELEMENTS   OF   PLANT   BIOLOGY 

practical  one,  conditioned  by  the  fact  that  the  students 
include  some  who  know  nothing  of  biology  and  are 
only  beginning  to  learn  elementary  chemistry  and 
physics,  and  by  the  further  fact  that  the  classes  are 
large,  varying  frorn  120  to  250  students,  and  have  to 
be  taught  on  the  old-fashioned  academic  system  of 
an  hour's  lecture  followed  by  two  hours'  practical 
work,  a  system  which  has  serious  educational  draw- 
backs, though  also  obvious  conveniences. 

The  problem,  then,  is  to  provide,  under  these 
conditions,  a  course  of  teaching  which  shall  be  as 
interesting  as  possible  and  shall  serve  to  introduce 
the  student  to  the  fundamental  facts  and  principles 
of  biology,  both  as  part  of  his  training  for  life  and 
more  particularly  as  an  introduction  to  the  study 
of  medicine,  which,  as  has  often  been  said,  is  really 
a  specialised  branch  of  applied  biology. 

The  time  is  past  when  we  could  hope  to  give  the 
budding  medical  student  a  thorough  training  in 
elementary  botany  and  zoology,  and  with  the  ever 
increasing  complexity  of  the  medical  curriculum 
proper  there  is  constant  pressure  to  shorten  and 
simplify  the  preparatory  parts.  It  seems  to  me  that 
biologists  should  yield  to  this  pressure  to  a  reasonable 
extent,  and  frankly  ask  themselves  how  far  the  things 
that  have  been  traditionally  taught  really  serve  as  the 
best  introduction  to  biology  for  medical  students  under 
existing  conditions. 

At  the  same  time  the  provision  of  a  good  intro- 
duction to  general  biology  in  the  early  part  of  the 
student's  course  is  certainly  of  as  much  importance 
now  as  ever  it  was.  If  the  foundations  are  not  broad 
enough,  the  student's  outlook  will  necessarily  suffer 
when  he  comes  to  his  more  specialised  work  ;  and 


PREFACE  9 

some  serious  knowledge  of  the  specific  structures, 
activities  and  evolution  of  plants  is  an  essential  part 
of  a  broad  biological  foundation.  There  are  things 
about  life  as  a  whole  that  we  can  only  learn  from 
plants,  and  there  are  others  which  some  knowledge 
of  plants  helps  us  to  understand  far  more  completely. 
The  ideal  course  in  elementary  biology  would  un- 
doubtedly be  given  by  a  teacher  dealing  with  both 
plants  and  animals.  Unfortunately,  the  differentiation 
of  botany  and  zoology  has  gone  much  too  far  to 
make  it  possible  to  find  or  to  employ  such  a  teacher 
in  most  existing  British  universities.  The  Cambridge 
course  attempts  a  compromise  which  approaches, 
though  it  cannot  attain,  this  ideal,  for  the  introductory 
lectures  deal  with  animals  as  well  as  with  plants, 
while  plants  as  well  as  animals  are  freely  used  as 
illustrations  in  the  concluding  lectures  of  the  zoological 
portion,  which  are  concerned  with  heredity  and 
evolution. 

If  all  school  children  were  taught  some  biology, 
as  they  certainly  should  be,  and  this  were  on  well 
considered  and  recognised  lines,  the  task  of  the 
university  teacher  would  be  more  straightforward 
and  more  profitable  :  he  could  give  his  students  a 
far  better  training  in  the  time  at  his  disposal.  As  it 
is  he  is  bound  to  assume  that  they  know  nothing 
of  biology  and  very  little  of  chemistry  and  physics. 
A  rudimentary  knowledge  of  inorganic  chemistry 
is,  however,  assumed  in  this  book,  and  this  is  now 
fairly  common  among  students  entering  the  university. 

The  chief  points  in  which  this  book  differs  from  most 
works  of  similar  scope  are  the  following.  First  of 
all  much  more  space  is  devoted  to  biological  facts 


10  ELEMENTS   OF   PLANT   BIOLOGY 

of  general  significance  which  can  be  best  illustrated 
by  the  lower  forms  of  plant  life.  The  vascular  plants 
are  briefly  treated  :  an  attempt  is  made  to  outline 
the  structure  and  life  history  of  the  seed  plant, 
but  much  of  the  morphology  is  left  on  one  side  as 
of  no  great  interest  or  importance  to  the  student 
of  general  biology.  Secondly,  after  an  introductory 
lecture  on  the  general  characters  and  differences 
of  animals  and  plants,  the  student  is  at  once  intro- 
duced to  the  most  important  organic  substances 
which  make  up  the  body  of  the  organism,  and  then 
to  a  brief  consideration  of  some  of  the  physical 
characters  of  organic  substances  and  of  protoplasm. 
This  is  followed  by  an  account  of  amoeba,  and  of  the 
chief  functions  of  organisms  in  general,  and  this  again  by 
a  general  account  of  the  cell  (Chapter  VI).  This  line  of 
approach,  entered  upon  before  the  student  knows 
anything  worth  mentioning  of  the  detailed  structure 
of  organisms,  certainly  has  drawbacks,  but  in  a 
necessarily  short  course  it  has  also  great  advantages, 
because  it  enables  the  structures  and  activities 
described  later  on  to  be  followed  with  some  under- 
standing of  their  significance  in  the  light  of  the  funda- 
mental properties  and  functions  of  organic  and  living 
substance. 

The  Green  Plant  cell,  as  the  most  fundamental 
unit  in  the  plant  world,  is  dealt  with  in  Chapter  VII, 
and  then  the  plant  cell  without  chlorophyll,  illustrated 
(as  functioning  organisms)  by  Yeast  and  Bacteria. 
The  Bacteria  are  considered  at  greater  length  than 
is  usual  in  elementary  books  of  this  scope,  because 
of  their  enormous  importance  in  the  general  economy 
of  the  world,  and  because,  as  it  seems  to  the  author, 
it  is  specially  desirable  for  medical  students  to  get 


PREFACE  II 

a  fair  general  notion  of  their  activities  before  they 
enter,  later  in  their  medical  course,  upon  the  highly 
specialised  study  of  the  disease-producing  forms — 
which  are,  after  all,  from  the  general  biological 
standpoint,  a  relatively  unimportant  fraction  of  the 
group.  The  saprophytic  and  parasitic  fungi  are  next 
dealt  with,  and  then  the  more  detailed  study  of 
the  chlorophyll-bearing  plants  is  entered  upon,  and 
occupies  the  rest  of  the  book.  Here  the  evolutionary 
series  is  roughly  followed,  for  it  seems  to  the  author 
that  the  interest  and  educational  advantage  of  this 
method  of  treatment  are  decisively  in  its  favour. 

The  Chlamydomonas-Volvox  series  is  dealt  with 
in  much  more  detail  than  is  usual,  because  of  the 
unique  interest  of  this  series  as  illustrating  the  origin 
of  sex  and  of  the  soma.  Fucus  is  taken  as  the  best 
example  of  primitive  tissue  differentiation  in  a  relatively 
large  plant,  Pellia  as  a  simple  land  plant.  The 
archegoniate  plants  are  treated  very  briefly  indeed, 
detailed  descriptions  of  structure  and  development 
being  deliberately  omitted,  though  an  attempt  is 
made  to  give,  in  very  brief  outline,  the  fundamental 
facts  of  the  biological  significance  of  vascular  plants 
and  of  heterospory. 

The  rest  of  the  book  deals  with  the  Seed  Plants, 
first  with  their  external  form  and  methods  of  vegetative 
propagation,  then  with  the  architectural  elements  of 
which  they  are  composed — a  course  which  has  been 
found  very  useful  in  practical  teaching  because  it 
relieves  the  later  descriptions  of  the  structure  of  the 
plant  organs  from  being  overloaded  with  accounts 
of  the  tissue  elements  themselves.  The  vegetative 
organs  of  the  plant — root,  leaf  and  stem — follow, 
and  the  account  of  the  vegetative  structure  of  the 


12  ELEMENTS   OF   PLANT   BIOLOGY 

seed  plant  closes  with  the  woody  stem.  Finally, 
the  flower,  fruit,  seed  and  seedling  are  dealt  with 
in  order,  thus  completing  the  sketch  of  the  structure, 
economy  and  life  history  of  the  seed  plant. 

Throughout  the  book  the  effort  has  been  made 
to  treat  the  material  from  the  point  of  view  of  general 
biology  rather  than  with  the  narrower  outlook  of 
pure  botany. 

The  schedules  of  practical  work  follow  pretty 
closely  those  in  use  in  the  Cambridge  course  ;  in  a 
few  cases  alternative  material  is  suggested.  Though 
it  is  not  suggested  that  the  selection  of  material 
cannot  be  improved,  nor  to  be  supposed  that  any 
teacher  will  desire  to  follow  the  schedules  in  every 
detail,  it  was  thought  that  the  interests  of  teachers  and 
students  would  best  be  served  by  presenting  a  perfectly 
definite  selection  of  material  which  has  been  proved 
by  experience  to  be  workable  and  instructive.  Section 
B  on  p.  34  and  (7)  on  p.  47  were  suggested  to  me  by 
Dr.  M.  C.  Rayner. 

The  practical  work  suggested  at  the  end  of  each 
chapter  occupies  from  2  to  2|  hours.  It  is  not  con- 
templated that  the  students  should  cut  their  own 
sections  in  that  time.  It  is  found  by  experience 
that  so  large  a  proportion  of  the  time  available  in  a 
short  course  is  spent  in  learning  to  cut  sections,  and 
the  sections  cut  are  often  so  inferior,  that  the 
result  is  wholly  unsatisfactory.  Much  better  results 
are  obtained  by  providing  students  with  good 
sections  and  then  insisting  that  they  shall  examine 
and  draw  them  properly.  It  is  not  of  course  suggested 
that  it  is  not  good  for  students  to  do  as  much  of  the 
necessary  manipulation  of  their  material  as  possible, 
but  the  particular  technique  of  cutting  hand-sections 


PREFACE  13 

with  a  razor  is  of  no  further  use  to  anyone  unless 
he  becomes  a  professional  botanist  (of  no  very  great 
use  indeed  to  many  professional  botanists),  and  to 
acquire  any  useful  proficiency  occupies  far  more  time 
than  it  is  worth,  when  the  student's  hours  are  strictly 
limited. 

With  a  small  class  and  ample  time  the  case  is  quite 
otherwise,  and  not  only  should  students  cut  many 
of  their  own  sections  under  these  conditions,  but 
they  may  be  taught  to  prepare  material  and  experi- 
ments with  great  advantage  to  themselves  and  to  their 
ultimate  grip  of  the  subject.  Such  conditions,  however, 
simply  do  not  exist  in  big  university  classes  with 
every  hour  of  the  student's  working  day  filled  up. 

In  the  earlier  schedules,  especially,  fairly  precise 
instructions  are  given  to  the  student,  and  some  general 
hints  are  prefixed  to  the  book  (p.  17),  suggested  by 
experience  of  the  mistakes  students  most  frequently 
make  in  doing  their  practical  work.  They  are  of 
course  intended  only  for  those  who  have  no  previous 
experience  of  microscopic  work  in  the  laboratory. 

It  is  believed  that  the  actual  measurement  of 
microscopic  objects,  though  not  usual  in  elementary 
classes,  is  a  valuable  help  to  the  student  in  relating 
the  structures  he  sees  under  the  microscope  to  the 
objects  he  sees  with  the  naked  eye,  giving  the  former 
a  greater  objective  reality  in  his  mind.  It  is  therefore 
suggested  that  the  student  should  learn  early  in 
the  course  to  calibrate  a  micrometer  eyepiece  with  a 
micrometer  slide,  and  that  he  should  frequently 
measure  the  diameters  of  microscopic  objects. 

I  am  greatly  indebted  to  my  daughter  Margaret, 
who  has  drawn  the  whole  of  the  illustrations.  The 


14  ELEMENTS   OF   PLANT   BIOLOGY 

majority  are  copied,  with  or  without  modification,  from 
published  figures.  They  are  intentionally  limited  in 
number,  no  attempt  having  been  made  to  illustrate 
everything  that  is  described  when  the  details  of 
structure  can  quite  easily  be  made  out  on  the  objects 
seen  in  the  laboratory.  Accurate  pictures  of  the 
material  actually  studied  provide  too  great  a  tempta- 
tion to  copy  the  picture  rather  than  the  object,  and 
it  is,  of  course,  impossible  to  lay  too  great  stress 
on  the  value  of  making  careful  drawings  from  the 
objects  themselves.  For  this  reason  a  good  deal  of 
the  structure  of  the  higher  plants  is  illustrated  by 
diagrams  and  generalised  pictures  which  are  not 
faithful  copies  of  nature. 

Professor  A.  E.  Boycott,  of  University  College, 
London,  has  been  good  enough  to  read  the  chapter 
on  Bacteria,  and  has  given  me  the  help  of  his  very 
valuable  criticisms.  My  colleague,  Mr.  F.  T.  Brooks, 
has  kindly  performed  a  similar  service  in  reading 
Chapters  X  and  XI,  on  the  Fungi.  Mr.  S.  M. 
Wadham,  the  Senior  Demonstrator  in  Botany  at 
Cambridge,  has  given  ungrudging  help  in  organising 
the  Practical  Work  and  suggesting  improvements. 
Finally,  I  am  deeply  indebted  to  my  friend  Dr.  F.  F. 
Blackman,  who  has  placed  at  my  disposal  his  critical 
knowledge  of  plant  physiology  and  biochemistry  by 
advising  in  detail  on  Chapters  II,  III  and  VII,  as  well 
as  on  other  smaller  sections  of  the  book. 

A.  G.  T. 

CAMBRIDGE, 
July,  1922. 


CONTENTS 

PAGE 

PREFACE -   .  •  -   --5 

HINTS  TO  STUDENTS  ON  PRACTICAL  WORK          .    - .        .  17 

CHAPTER 

I.     INTRODUCTORY.     PLANTS  AND  ANIMALS        .        .  21 
II.     ORGANIC     SUBSTANCES    AND     THEIR     CHEMICAL 

CHARACTERS '     .  .    .       .  36 

III.  SOME  PHYSICAL  CHARACTERS  OF  ORGANIC   SUB- 

STANCES        ...       .       f.       .'.       .       .  48 

IV.  PROTOPLASM  AND  THE  AMCEBA.     PROTOCOCCUS     .  61 
V.     THE  VITAL  FUNCTIONS       .               ....  76 

VI.     THE  CELL 92 

VII.  THE  GREEN  PLANT  CELL 112 

VIII.  THE  COLOURLESS  PLANT  CELL.   THE  YEAST 

PLANT 127 

IX.     BACTERIA 138 

X.     SAPROPHYTIC  FUNGI.     MUCOR  AND  PENICILLIUM.  157 

XI.     PARASITIC  FUNGI        .       .       .       .       .       .       .  169 

XII.     ORIGIN    OF    SEX   AND    OF    THE    SOMA.         THE 

GREEN  ALG^E       .        .        .       .       «       '.       .  184 

XIII.     DIFFERENTIATION    OF    TISSUES.      Fucus :     THE 

SEA  WRACK         .......  214 

15 


l6  CONTENTS 

CHAPTER  FACE 

XIV.     THE     SIMPLEST     LAND      PLANTS — LIVERWORTS 

AND  MOSSES.     THE   PTERIDOPHYTA       .       .231 

XV.     THE  SEED  PLANTS  :   FORMS  AND  LIFE  HISTORIES  254 

XVI.      THE  TISSUE  ELEMENTS  OF  SEED  PLANTS      .        .  270 

XVII.    THE  ROOT     .       .       .       .       .       .       .       .       .288 

XVIII.     THE  FOLIAGE  LEAF 302 

XIX.     THE  PRIMARY  STEM 315 

XX.     THE  WOODY  STEM 328 

XXI.     THE  FLOWER 344 

XXII.     THE   FRUIT 361 

XXIII.     THE  SEED  AND  ITS  GERMINATION    .       .       .       .  375 

XXIV.     CONCLUSION 389 

INDEX 405 


HINTS    TO    STUDENTS    ON    PRACTICAL    WORK 

Use  of  the  Microscope. 

(1)  Remember  that  the  microscope  is  a  delicately 
adjusted  scientific  instrument,  and  must  not  be  treated 
roughly.     Do  not  lift  it  by  the  tube,  but  always  by 
the  solid  part  of  the  stand. 

(2)  Take  great  care  not  to  get  liquids  of  any  kind 
on  to  the  lenses.     If  a  lens  is  dusty,  wipe  it  with  a  soft 
handkerchief. 

(3)  Always    see   that   the   slide    and    coverslip    are 
perfectly  clean  before  use. 

(4)  If  you  cannot  then  get  a  clear  view  of  an  object, 
look  first  to  the  surfaces  of  the  lenses,  especially  of  the 
objectives.     If  there  is  glycerine  on  the  surface  of  the 
lens,  it  must  be  carefully  and  repeatedly  wiped  with 
a  soft  handkerchief.     Breathing  on  the  lens  between 
wipings  will  often  help.     Take  care  never  to  scratch 
the  lenses.     This  is  often  done  by  rubbing  with  a  harsh 
cloth. 

(5)  In   examining   an   object   or   section   under   the 
microscope  look  at  it  first  with  the  naked  eye,  then 
with  a  hand  lens,  then  with  the  low  power,  and  finally 
with  the  high  power.     This  procedure  enables  you  to 
get  a  preliminary  idea  of  the  general  form  of  the  object 
or  section,  before  you  look  at  its  details,  and  thus  to 
relate  the  details  to  the  larger  features  of  its  structure. 
Remember  that  the  world  you  see  under  the  micro- 
scope is  not  really  a  different  world  from  that  which 

2  I7 


l8  HINTS   TO   STUDENTS    ON    PRACTICAL    WORK 

you  see  with  the  naked  eye.  The  microscope  is  merely 
an  instrument  which  extends  the  power  of  the  eye 
so  that  it  can  detect  the  fine  details  of  structure. 

(6)  Never  put  on  the  high  power  unless  the  object 
is  covered  with  a  coverslip,  or  you  will  be  almost  certain 
to  get  the  mounting  medium  on  to  the  lens  of  the 
objective. 

(7)  Never  focus  the  high  power  carelessly,   or  you 
will  jam  it  on  to  the  slide,  spoil  the  object,  and  very 
likely  injure  the  objective.     When  you  have  put  on 
the  high  power,  note  its  distance  from  the  coverslip 
by  looking  from  the  side  (you  will  soon  learn  to  judge 
about  the  distance  at  which  it  is  in  focus).     Correct 
the  distance  with  the  coarse  adjustment  carefully,  till 
you  judge  it  is  about  right,  and  then  look  through  the 
microscope  and  focus  accurately  with  the  fine  adjust- 
ment. 

Observation  and  Drawing  of  Structure. 

Careful  observation,  followed  by  accurate  drawing 
of  the  structures  of  plants  and  animals,  whether  seen 
with  the  naked  eye,  with  a  hand  lens  or  under  the 
microscope,  is  an  indispensable  means  of  acquiring  a 
sound  first-hand  knowledge  of  biology.  It  is  also  one  of 
the  best  methods  of  training  the  mind  to  form  and  retain 
clear  mental  pictures,  and  this  power  is  most  valuable 
in  almost  every  occupation  of  life,  most  notably  in  the 
everyday  work  of  the  medical  man.  It  is  therefore  of 
great  importance  that  the  student  should  seriously 
bend  his  mind  from  the  start  to  learning  how  to 
represent  what  he  sees  in  drawings  and  diagrams. 

The  power  of  making  good  representations  on  paper 
of  what  one  sees  varies  very  much  in  different  people 
— some  find  it  comparatively  easy,  others  very  difficult. 


DRAWING  19 

But  those  students  who  find  it  difficult  should  not  be 
discouraged,  for  everyone  can  improve  this  power  by 
practice,  and  it  is  the  effort  to  improve  which  is  specially 
valuable  as  training  of  the  mind. 

The    following    points    should    always    be    kept    in 
view : — 

(1)  Outlines    should    be    represented    by    clear  firm 
lines  on  smooth  paper  with  a  pencil  of  medium  hardness 
sharpened  to   a  good  point.     Avoid   all   "  muzziness  " 
in  drawings  :    it  always  indicates  a  "  muzzy  "  mental 
picture,  which  is  useless.     Do  not  put  in  "  shading  " 
in  drawings  of  microscopic  objects,  and  never  "  shade  " 
any  drawing  till  you  are  quite  satisfied  that  the  outlines 
are  accurate.     Shading  is  very  seldom  necessary. 

(2)  Draw  only  what  you  can  actually  see.     Never 
pretend  to  see  what  you  do  not  see.     But  remember 
that   practice  in   observation   will   enable  you   to   see 
things  that  you  cannot  see  at  first.     So  do  not  jump 
to  the  conclusion  that  things  cannot  be  seen  which 
the  demonstrator  says  can  be  seen,  because  you  cannot 
see  them  at  once. 

(3)  Always  draw  on  a  large  scale,  so  that  all  details 
can  be  clearly  shown. 

(4)  Always  write  the  names  of  the  parts  of  what 
you  draw  at  the  side  of  the  drawing,  either  in  full  or 
with  unmistakable  abbreviations,  connecting  each  name 
with  the  corresponding  part  by  a  straight  line,  so  that 
the  drawing  is  at  once  intelligible. 

(5)  When    you    have    to    represent    a    complicated 
structure  as  seen  under   the  microscope,  for  instance 
a  section  of  an  organ  of  a  higher  plant  showing  various 
tissues,    make    (a)    a   diagram,   including  the    outlines 
only  of  the  tissues,  under  the  low  power,  to  show  the 
distribution  of  the  tissues  ;    and  (6)  detailed  drawings 


20  HINTS   TO   STUDENTS    ON    PRACTICAL    WORK 

of  small  samples  of  each  tissue  under  the  high  power, 
to  show  the  structure  of  the  different  cells. 

Do  not  make  a  drawing  of  the  whole  section  and 
try  to  put  in  all  the  cells.  To  do  it  accurately  would 
take  a  very  long  time,  and  the  time  spent  would  be 
largely  wasted.  To  do  it  carelessly  and  inaccurately 
is  worse  than  waste  of  time  :  it  is  training  in  slovenly 
work. 


Elements  of  Plant  Biology 

CHAPTER   I 
INTRODUCTORY.      PLANTS   AND    ANIMALS 

BOTANY  is  that  part  of  biology  specially  concerned 
with  those  living  organisms  we  call  plants.  Plants 
form  the  basis  of  all  life  as  it  is  lived  upon  the  earth, 
because  they  alone  have  the  power  of  making  new 
supplies  of  what  are  called  organic  substances — the 
only  substances  on  which  animals  can  feed — out  of 
inorganic  substances.  And  the  study  of  plants  teaches 
us  some  things  about  life  which  we  cannot  learn, 
and  others  which  we  cannot  so  easily  learn,  from  the 
study  of  animals.  It  is  therefore  necessary  not  only 
for  all  those  who  want  to  acquire  some  knowledge  of 
biology  as  a  whole,  but  also  for  those  who  will  be  daily 
concerned  in  their  profession  with  the  most  highly 
developed  and  most  complex  of  all  living  organisms 
— man — to  learn  something  about  plants  if  they  are 
to  acquire  a  firm  foundation  for  their  later  studies. 
It  is  not  necessary  for  them  to  gain  the  detailed  and 
comprehensive  knowledge  that  the  botanist  has  to 
acquire,  and  their  studies  should  be  directed  rather 
to  what  plants  can  teach  them  about  life  as  a  whole 
than  to  a  knowledge  of  plants  for  their  own  sakes. 
The  title  of  this  book,  which  is  designed  especially 
for  the  medical  student  who  knows  nothing  of  biology, 


22  INTRODUCTORY.      PLANTS   AND  ANIMALS 

and  whose  time  is  strictly  limited,  is  intended  to  empha- 
sise this  point  of  view.  But  such  a  student  must  under- 
stand that  if  he  is  to  acquire  a  foundation  which  shall 
be  of  real  use  to  him  in  his  later  work  he  must  give 
himself  freely  to  the  labour  of  acquiring  a  firm  elemen- 
tary knowledge  of  the  nature  and  constitution  of  the 
bodies  of  plants,  of  the  outline  of  their  structures  and 
of  their  various  activities,  not  only  from  books  and 
lectures,  but  also  at  first-hand  in  the  laboratory. 

What  is  a  Plant  ?  Differences  between  Animals  and 
Plants. — The  animals  and  plants  with  which  we  are 
most  familiar  are  the  higher,  i.e.  the  more  complex 
ones.  They  mostly  live  on  the  land,  and  are  on  the 
whole,  though  by  no  means  invariably,  bigger  than 
the  lower,  less  complex,  forms.  Certain  outstanding 
differences  between  the  familiar  higher  animals,  on 
the  one  hand,  and  the  familiar  higher  plants,  on  the 
other,  are  very  obvious.  The  animals  move  about, 
the  plants  are  rooted  in  one  place  :  the  animals  are 
compact,  the  plants  are  branching,  in  their  habit  of 
body  :  the  animals  are  variously  coloured,  the  plants 
are  green,  or  at  least  have  green  leaves.  The  most 
important  functional  difference  is  that  animals  consume 
solid  food,  while  plants  do  not  ;  and  this  is  really  the 
root  of  all  the  other  differences. 

The  locomotion  of  animals  is  related  to  the  fact 
that  they  have  to  move  about  to  find  their  food,  which 
must  be  organic,  i.e.  must  consist  of  special  complex 
chemical  substances  forming  part  of,  or  produced  by, 
other  animals  or  plants,  while  the  food  of  plants  consists 
of  liquid  and  gaseous  inorganic  substances  which  are 
found  everywhere  in  the  earth  and  air. 

The  compact  form  of  animals  is  connected  with  the 
necessity  of  locomotion  and  with  the  fact  that  the 


DIFFERENCES   BETWEEN   ANIMALS   AND   PLANTS       23 

body  is  organised  round  the  gut  (alimentary  canal), 
which  digests  the  organic  food,  i.e.  changes  it  into 
forms  which  can  directly  nourish  the  body.  The 
branching  habit  of  plants,  on  the  other  hand,  exposes 
the  greatest  possible  surface  to  the  soil  and  to  the  air, 
from  which  the  plant  directly  absorbs  the  liquid  and 
gaseous  inorganic  substances  which  form  its  food. 

Plants  are  green  or  have  green  parts  because  of 
the  green  colouring  matter  (chlorophyll l)  which  enables 
them  to  build  up  their  bodies  from  simple  inorganic 
substances — or,  in  other  words,  to  form  organic 
substances  from  inorganic. 

And  the  many  other  differences  of  structure  and 
organisation  which  distinguish  the  higher  animals  and 
the  higher  plants  are  all  related,  directly  or  indirectly, 
to  the  fundamental  difference  in  their  foods. 

There  are,  however,  cases  in  which  both  animals 
and  plants  depart,  in  one  or  other  respect,  from  these 
general  characters.  There  are  parasitic  animals  which 
do  not  move  about,  but  remain  fixed  on  or  in  the 
"  host,"  and  which  consume  only  liquid  food,  though 
this  is  always  organic,  and  derived  from  the  body  of 
the  host.  There  are  plants  which  are  partly  carnivorous, 
with  special  organs  which  digest  solid  animal  food 
and  absorb  the  products  of  digestion.  There  are 
branching  animals  and  compact  plants.  There  are 
plants  which  live  largely  on  organic  (liquid)  food. 

Furthermore,  there  are  simple  minute  microscopic 
organisms  living  in  water  and  other  liquids  which 
have  some  animal  and  some  plant  characters,  so  that 
biologists  have  sometimes  considered  them  as  animals 
and  sometimes  as  plants.  These  last  are  more  or  less 
changed  representatives  of  some  of  the  organisms 

1  "  Leaf-green,"  from  Greek  ^Acopdg,  green,  and  ^v'AAov,  a  leaf. 


24       INTRODUCTORY.   PLANTS  AND  ANIMALS 

which  were  produced  early  in  the  history  of  life,  and 
they  have  not  become  differentiated  like  the  higher 
organisms,  the  unmistakable  familiar  animals  and 
plants.  Thus  there  is  a  minute  oval  organism  called 
Chlamydomonas  (Fig.  20,  p.  185)  which  lives  in  pools. 
It  is  green  and  lives  exclusively  on  liquid  and  gaseous 
food  like  a  plant,  but  during  most  of  its  life  it  swims 
actively  about  like  an  aquatic  animal.  It  must  be 
reckoned  as  a  plant,  because,  as  we  saw,  the  mode  of 
feeding  is  the  basal  character  of  difference,  but  it  still 
retains  the  animal  character  of  free  locomotion.  And 
there  are  other  minute  organisms  which  feed  in  both 
ways,  partly  on  liquid  and  gaseous  inorganic  food, 
partly  on  solid  organic  food,  and  these  escape  the 
meshes  of  any  definition  of  an  animal  or  of  a  plant. 

We  cannot,  in  fact,  frame  any  comprehensive  definition 
which  shall  sharply  separate  all  living  organisms  into 
animals  and  plants.  This  conclusion  illustrates  a 
truth  that  the  student  will  realise  more  and  more  fully 
as  his  biological  studies  progress,  namely,  that  it  is 
useless  to  expect  the  facts  of  nature  exactly  to  fit  our 
definitions,  however  carefully  framed.  There  are  so 
many  different  ways  in  which  living  substances  may 
take  shape,  so  many  varieties  of  function,  i.e.  of  ways 
in  which  it  may  work,  and  so  many  possible  combina- 
tions of  both  of  these,  that  there  are  certain  to  be  excep- 
tions to  every  rule  we  try  to  lay  down.  'But  that  does 
not  mean  that  we  have  to  give  up  the  task  of  analysing 
and  classifying  form  and  function,  or  the  science  of 
biology  would  be  impossible.  We  can  always  recognise 
types  of  structure  and  types  of  function  to  which  organisms 
conform,  more  or  less  closely,  because  their  physical 
and  chemical  constitution  forces  them,  so  to  speak, 
along  certain  lines  of  differentiation  and  behaviour. 


RELATION   TO   ENERGY  25 

Only  we  must  not  expect  to  draw  sharp  lines.  The 
forms  included  in  any  groups  we  make  will  always 
tend  to  shade  off  into  forms  belonging  to  other  groups. 

Thus  we  can  say  that  the  essential  animal  character 
depends  upon  the  habit  of  consuming  solid  organic 
food  ;  that  the  essential  plant  character  depends  upon 
the  habit  of  absorbing  liquid  and  gaseous  inorganic 
food ;  that  a  man  is  indubitably  an  animal,  while 
an  oak  tree  is  as  indubitably  a  plant ;  that  with  the 
man  we  can  group  a  host  of  other  different  kinds  of 
organism  as  animals,  and  with  the  oak  tree  a  host  of 
other  organisms  as  plants  ;  but  that  when  we  come 
to  minute  microscopic  organisms  we  find  the  differ- 
ences which  are  so  clear  and  sharp  among  the  higher 
forms  becoming  blurred,  till  finally  we  arrive  at  forms 
of  which  it  is  impossible  to  assert  that  they  are  definitely 
animals  or  definitely  plants. 

Relation  of  Animals  and  Plants  to  Energy. — The 
essential  difference  between  animals  and  plants  in 
their  relation  to  food  determines  also  their  character- 
istic difference  in  relation  to  energy.  All  living  organisms 
may  be  regarded  as  machines  transforming  energy 
from  one  form  into  another,  for  instance,  from  the 
potential  energy  locked  up  in  the  molecules  of  organic 
food  to  the  kinetic  energy  seen  in  motion  of  the  body 
and  in  the  production  of  heat  (animal),  or  from  the 
"  radiant  "  (kinetic)  energy  of  sunlight  to  the  potential 
energy  of  organic  substances  formed  in  the  body 
(green  plant).  This  subject  will  be  dealt  with  more 
in  detail  in  later  chapters. 

Meanwhile  we  note  the  broad  difference  between 
animals  and  plants  in  their  relation  to  energy  is  that 
animals  consume  organic  food  and  spend  the  energy  it 
contains  in  heat  and  motion,  while  plants  build  organic 


26      INTRODUCTORY.   PLANTS  AND  ANIMALS 

substance  and  store  energy.  And  since  life  consists  in  the 
expenditure  of  energy,  animals  live  much  more  intensely 
than  plants,  and  correspondingly  they  are  sensitive  to 
much  more  varied  stimuli,  i.e.  to  the  influences  which 
lead  to  their  expending  energy  in  definite  ways.  But 
because  they  are  alive  plants  do  spend  energy  and  are 
sensitive  to  stimuli.  In  the  course  of  the  life  of  a  tree, 
for  instance,  a  large  amount  of  energy  is  expended  in 
lifting  the  branches  high  into  the  air  and  in  pushing 
the  root  tips  through  the  soil,  though  these  things 
are  done  so  slowly,  measured  by  the  standard  of  the 
rate  of  motion  in  animals,  that  we  are  not  at  once 
impressed  by  them.  The  shoots  of  a  plant  also  bend 
towards  the  light,  and  the  roots  towards  supplies  of 
water,  i.e.  they  respond  to  different  stimuli  in  definite 
ways.  Also  animals  do  store  energy  in  various  forms 
of  organic  food,  e.g.  glycogen  (a  form  of  starch)  in  the 
liver  and  the  muscles,  and  thus  have  a  reserve  supply 
of  potential  energy  which  enables  them  to  carry  on 
their  active  life  for  a  time  without  consuming  fresh  food. 
The  Naming  and  Classification  of  Plants  and  Animals. 
— The  immense  multitude  of  individual  plants  and 
animals  existing  on  the  earth  are  exceedingly  varied 
in  form  and  structure,  and  in  order  to  get  any  under- 
standing of  plant  or  animal  life  we  must  classify  them 
in  some  way.  We  recognise  at  once — mankind  has 
recognised  from  the  earliest  times — that  there  are 
many  different  kinds,  but  of  these  some  resemble  each 
other  so  closely  that  they  are  difficult  to  distinguish, 
and  can  only  be  separated  by  those  who  have  made 
a  special  study  of  the  forms  in  question,  while  others 
are  very  distinct  indeed.  For  instance,  a  blackberry 
is  obviously  different  from  a  raspberry  in  the  colour 
and  taste  of  the  fruit,  and  in  the  fact  that  the  former 


CLASSIFICATION  27 

has  hard  prickles  which  can  tear  the  flesh,  while  the 
latter  has  soft  prickles  which  cannot.  But  there  are 
many  different  kinds  of  blackberry,  some  of  which 
differ  from  one  another  in  such  small  and  variable 
characters  that  even  specialists  who  have  spent  a  large 
part  of  their  lives  in  the  study  of  the  blackberries  do 
not  agree  as  to  exactly  how  they  should  be  grouped. 

We  distinguish  the  kinds  or  groups  of  individuals 
which  resemble  one  another  more  or  less  closely,  and 
which  interbreed  freely,  as  species,  but  authorities 
differ  as  to  what  they  consider  species,  and  there  is 
not  yet  agreement  as  to  whether  the  conception  of  a 
species  can  be  made  at  all  a  precise  conception.  Species 
which  are  most  like  each  other  are  grouped  into  genera, 
and  genera  which  are  most  like  each  other  into  families. 
Thus  the  raspberry  and  the  blackberry  are  different 
species  of  one  genus,  the  sweet  briar  and  the  dog  rose 
of  another  genus  belonging  to  the  same  family.  The 
hare  and  the  rabbit  are  different  species  of  one  genus  ; 
the  dog,  the  wolf  and  the  fox  of  another  belonging  to 
quite  a  different  family,  though  to  the  same  large 
group,  the  mammals,  which  include  all  animals  that 
suckle  their  young,  comprising  such  diverse  types  as 
rats  and  mice,  elephants,  whales  and  men. 

The  conventional  nomenclature,  which  is  used  in 
naming  the  different  species  we  recognise,  and  enables 
us  to  record  and  systematise  them,  gives  each  genus 
a  Latin  name — a  noun — and  adds  a  qualifying  adjec- 
tive for  the  species.  Thus  the  genus  to  which  both 
the  blackberry  and  raspberry  belong  is  called  Rubus, 
the  former  (if  we  lump  all  the  blackberries  together  as 
one  species)  being  Rubus  fruticosus,  the  latter  Rubus 
idceus.  The  genus  Rosa  includes  Rosa  rubiginosa,  the 
sweet  briar,  and  Rosa  canina,  the  dog  rose,  as  well 


28  INTRODUCTORY.      PLANTS   AND   ANIMALS 

as  many  other  species.  Both  these  genera,  along  with 
many  others,  belong  to  one  family,  the  Rosacece,  or 
Rose  family,  because  they  have  certain  characters  of 
the  flower  and  fruit  in  common.  The  hare  is  Lepus 
timidus,  the  rabbit  Lepus  caniculus :  the  dog,  the 
European  wolf  and  the  fox  respectively,  Oanisfamiliaris, 
C.  lupus,  and  C.  vulpes.  In  the  last  two  cases  it  will 
be  seen  that  the  specific  name  is  a  noun  (the  Latin 
name  of  the  animal  in  question),  though  it  is  used  in 
place  of  an  adjective.  Very  many  species,  of  course, 
have  no  common  names  in  any  language  because  they 
have  not  impressed  their  existence  on  man  by  their 
usefulness  or  harmfulness,  or  conspicuousness — they 
have  not  attracted  his  attention  in  any  way,  until 
botanists  and  zoologists  began  to  study  the  different 
forms  for  their  own  sakes.  The  Latin  nomenclature 
has  the  indispensable  advantage  of  being  international. 
It  is  in  fact  a  relic  of  the  time  when  Latin  was  the 
universal  language  of  learned  men. 

Genera,  as  has  been  said,  are  grouped  into  families, 
families  into  orders,  orders  into  still  larger  groups, 
each  successively  higher  grade  of  groupings  containing 
forms  which  are  less  and  less  like  those  belonging  to 
other  groupings  of  the  same  grade. 

Range  of  Form  and  Structure  in  the  Plant  World. 
— Starting  with  the  higher  forms,  including  the  trees, 
shrubs  and  herbs  with  which  we  are  most  familiar, 
the  following  are  the  larger  groups  of  plants  : 

(i)  The  SEED  PLANTS  (SPERMOPHYTES),  including 
(i)  the  Angiosperms,  or  true  Flowering  Plants,  which 
produce  seeds  completely  enclosed  in  bag-  or  box-like 
structures,  and  comprise  nearly  all  the  herbs,  grasses, 
etc.,  and  the  shrubs  and  broad-leaved  trees,  and  (ii) 
the  Gymnosperms,  with  seeds  not  so  enclosed,  but 


THE   LARGER   GROUPS   OF   PLANTS  29 

often  borne  in  cones,  as  in  the  Coniferse,  mostly  needle- 
leaved  trees  (pines,  firs,  etc.).  The  Seed  Plants  have 
the  most  complicated  internal  structure  of  any  plants. 

(2)  The  PTERIDOPHYTES,  plants  of  the  same  grade 
of  organisation  as  the  Ferns,  and  possessing,  like  the 
Seed  Plants,  stems,  roots  and  leaves,  and  a  well-devel- 
oped internal  water-conducting  system,  but  reproducing 
themselves  not  by  seeds,  but  by  very  minute  bodies 
called   spores.     These  include   the  Ferns,   Olubmosses, 
and   Horsetails.     The   living   forms    of   the    two    last- 
named  groups  represent   a  very   small   proportion   of 
the  plants  belonging  to  these  groups,  which  flourished 
in  certain  past  ages,  and  which,  often  growing  to  the 
size  of  trees,  are  preserved  as  fossils  in  the  coal  measures 
and   other  rocks.     In   the   same  rocks   are  preserved 
forms  which  are  intermediate  between  the  Pteridophytes 
and  the  Seed  Plants.     These  formed  seeds,  but  seeds 
which  were  different  in  structure  from  the  seeds  of 
existing  plants. 

(3)  The  BRYOPHYTES,  including  the  Mosses  and  the 
Liverworts,  also  reproducing  themselves  by  spores,  but 
much  smaller  on  the  whole  than  the  Pteridophytes, 
and  with  a  less  well-developed  water-conducting  system, 
or  none  at  all.     The  Mosses  and  some  of  the  Liverworts 
have  distinct  stems  and  leaves,  but  other  liverworts 
have  none,  the  plant  body  consisting  of  a  flat  thallus, 
usually    a  branched    ribbon-shaped    structure,    rather 
like    an    indefinitely    growing    leaf.     These    forms    are 
mostly  only  able  to  live  in   relatively  damp  places, 
though  the  majority  are  land  plants. 

(4)  The  ALG^J,  plants  living  mostly  in  water,  and 
able,    like    the    previous  groups,  to  form  organic  sub- 
stances from  inorganic.     They  all  contain  chlorophyll, 
but  some  of  them,  the  red  and  brown  seaweeds,  for 


30  INTRODUCTORY.      PLANTS  AND  ANIMALS 

instance,  are  not  green  in  colour,  because  the  colour 
of  the  chlorophyll  is  modified  by  the  presence  of  other 
pigments.  They  include  the  seaweeds,  many  of  which 
are  large  plants,  with  a  considerable  degree  of  internal 
organisation,  and  the  alga  that  live  in  freshwater, 
most  of  which  are  pure  green  in  colour,  and  consist  of 
simple  filaments  or  single  microscopic  cells  (see  below). 

(5)  The  FUNGI,  plants  of  about  the  same  grade  of 
organisation  as  the  algae,  and,  like  them,  comprising 
a  great  range  of  size  and  complexity  of  organisation, 
from  the  bulky  mushrooms  and  toadstools  to  simple 
moulds   and  mildews   and  minute  microscopic   forms. 
They  are  distinguished  by  not  possessing  chlorophyll, 
and  they  depend  on  organic  food. 

(6)  The   LICHENS,   peculiar  compound  plants,   each 
consisting  of  an  alga  and  a  fungus  associated  together 
in  one  plant  body,  or  thallus,   which  resembles  in  a 
general  way   the  liverwort  thallus  referred  to   above, 
though  it  often  branches  in  a  complicated  way.     The 
thallus   of  lichens  is   variously   coloured  :     sometimes 
the  green  colour  of  the  included  alga  can  be  seen  through 
the  fungal  investment.     Lichens  are  especially  found 
on  rocks,  the  trunks  of  trees,   or  the  bare  soil. 

Many  of  the  algae  and  fungi  are  microscopic,  and  some 
of  them  are  very  minute  indeed.  Among  these  micro- 
scopic forms,  which  live  in  water  or  some  other  liquid, 
we  come  to  the  groups  of  organisms  referred  to  on 
pp.  23-4,  which  are  on  the  borderline  between  plants 
and  animals,  and  are  sometimes  classed  together  as 

(7)  The  PROTISTA.    Among  these  are  the  BACTERIA, 
the  smallest  of  all  visible  organisms,  which  are  on  the 
whole  plants,  but  many  have  the  animal  character  of 
locomotion.     They    are    of    enormous    importance    to 
mankind,   as   we   shall   see  in   a   later  chapter.     The 


CELLULAR   STRUCTURE  3! 

Protista  include  many  other  groups  of  simple  organisms 
of  very  various  characters. 

As  we  descend  the  series  outlined  above,  starting 
with  the  higher  (most  complex)  plants,  we  find  that 
the  forms  of  plant  life  (with  the  exception  of  the  fungi 
and  lichens)  are  more  and  more  dependent  on  external 
water,  most  of  the  algae  living  all  their  life  immersed 
in  water.  It  is  established  biological  doctrine  that 
the  higher  plants,  which  are  able  to  live  on  dry  land, 
though  their  roots  must  in  fact  always  be  able  to 
obtain  some  water  from  the  soil,  have  gradually  arisen, 
during  the  history  of  the  world,  from  forms  like  these 
lower  plants  which  are  confined  to  water.  This  is 
the  doctrine  of  organic  evolution,  as  applied  to  plants, 
a  doctrine  of  whose  truth  Darwin  first  succeeded  in 
convincing  the  world. 

Cellular  Structure  of  Plants. — When  we  come  to 
examine  their  structure  with  the  microscope,  we  find 
that  the  bodies  of  nearly  all  plants  and  animals  are 
composed  of  what  are  called  cells  and  the  products 
of  cells.  In  the  case  of  plants  the  cells  are  closed 
spaces,  surrounded  by  a  cell  wall  of  non-living  substance, 
and  containing  living  substance  called  protoplasm,  in 
which  all  the  ultimate  life  processes  occur.  This  proto- 
plasm is  the  only  part  of  the  bodies  of  living  beings 
which  is  actually  alive.  A  large  part  of  the  body 
consists  of  non-living  organic  substances  formed  by 
the  protoplasm.  Thus  in  plants  the  body  is  composed 
of  a  "  skeleton  "  of  cell  walls,  with  living  substance 
in  each  cell  cavity.  The  protoplasm  is,  however, 
generally  continuous  from  cell  to  cell  by  means  of 
exceedingly  thin  filaments  of  protoplasm  (which  cannot 
be  seen  by  ordinary  microscopic  observation)  passing 


32       INTRODUCTORY.   PLANTS  AND  ANIMALS 

through  the  cell  walls,  so  that  all,  or  nearly  all,  the 
protoplasm  of  the  plant  really  forms  a  single  connected 
structure. 

The  protoplasm  of  many  of  the  cells  of  the  bodies 
of  the  higher  plants  disappears  during  the  life  of  the 
plant,  so  that  only  the  dead  cell  walls  remain,  for  in- 
stance in  the  outer  bark  and  the  heartwood  of  tree 
trunks,  which  are  thus  dead,  though  integral  parts  of 
the  living  plant  body,  just  as  are  the  nails,  for  instance, 
in  the  case  of  man. 

Green  Colour  of  Plants.  Chlorophyll.  —  The  great 
majority  of  plants,  from  the  simplest  algae  to  the 
flowering  plants,  are  green,  or  have  green  parts  (fungi 
and  bacteria  are  an  exception).  It  has  already  been 
mentioned  that  this  green  colour  is  due  to  the  presence 
of  a  pigment  called  chlorophyll,  whose  presence,  owing 
to  the  light  which  it  absorbs,  enables  the  protoplasm 
of  the  plant  cell  containing  it  to  build  up  organic 
substances  out  of  simple  inorganic  substances  which 
the  cell  absorbs.  The  chlorophyll  is  contained  in 
definite  protoplasmic  bodies  called  chloroplasts,  usually 
spherical,  oval  or  disc  shaped,  within  the  general 
protoplasm  of  the  cell.  This  process  of  the  formation 
of  organic  substances  from  inorganic  by  the  activity 
of  the  chloroplasts  under  the  action  of  light  is  called 
photosynthesis.*  It  is  upon  photosynthesis  that  the  con- 
tinuance of  life  in  the  world  ultimately  depends,  because 
it  is  the  only  natural  process  which  makes  new  organic 
substance  on  a  sufficiently  large  scale. 

Protoplasm,  as  has  been  said,  is  the  seat  of  all  the 
essential  life  processes.  Before  we  can  begin  to  analyse 
the  nature  and  relations  of  these  it  is  essential  to  learn 
something  of  the  nature  of  protoplasm  itself.  Though 

1  "  A  putting  together  with  the  help  of  light,"  from  Greek  </>a>;, 
<f>a)TOQ,  light,  and  avvdeaig,  putting  together. 


PRACTICAL   WORK  33 

we  are  far  from  understanding  the  whole  secret  of  life, 
we  do  know  that  many  of  the  manifestations  of  life 
depend  upon  the  chemical  nature  and  physical  proper- 
ties of  protoplasm  and  of  the  substances  derived  from 
it.  The  next  two  chapters  will  therefore  be  devoted  to 
these  subjects. 

PRACTICAL  WORK. 

A.     USE  OF  THE  MICROSCOPE. 

(1)  Examine  the  microscope,  especially  the  mirror,  substage 
diaphragm,  low  and  high  power  objectives,  eyepieces,  coarse  and 
fine  focussing  adjustments.     [The  demonstrator  should  show  the 
students  how  to  handle  a  microscope,  explain  the  outlines  of  its 
working,  and  give  the  necessary  cautions.] 

(2)  Put  a  small  drop  of  water  on  the  middle  of  a  slide,  and 
place  in  it  a  few  grains  of  sand.     Cover  with  a  coverslip  so  as 
to  include  in  the  water  some  bubbles  of  air. 

Examine  first  with  the  low  power  and  then  with  the  high 
power 

(a)  a  sand  grain,  noting  the  shape,  colour  and  translucency  ; 

(b)  an  air  bubble,  noting  its  appearance  in  different  focal 

planes. 

(3)  Mount    a    single    leaf   from    a  moss  plant  in  a  drop  of 
water.    Cover  with  a  coverslip    so    as  not  to    include   bubbles 
of  air. 

Examine  first  with  the  low  and  then  with  the  high  power. 
Note  that  the  leaf  consists  of  a  single  layer  of  box-like  compart- 
ments (cells),  with  transparent  walls,  through  which  can  be  seen 
the  green  chloroplasts  containing  the  pigment  chlorophyll.  The 
cells  of  the  moss  leaf  are  a  good  type  of  green  plant  cell. 

(4)  Determine  the  apparent  length  in  ^  x  in  the  field  of  the 
microscope  (with  fixed  tube  length)  of  a  division  of  the  eyepiece 
micrometer  scale,   (a)  with  the  high  and  (b)  with  the  low  power 
objective,  by  determining  the    correspondence   of  the    eyepiece 
scale  with  the  divisions   of  the  measured  scale  engraved  on  the 
micrometer  slide.     Make  a  note  of  these  values  for  future  use. 
Now  measure  the  length  and  breadth  of  a  cell  of  the  moss  leaf  and 
the  diameter  of  a  chloroplast  under  the  high  power. 

1  n  =  -ooi  mm. 

3 


34  INTRODUCTORY.      PLANTS  AND  ANIMALS 

B.     TYPES  OF  THE  PLANT  KINGDOM. 

A  series  of  specimens  will  be  given  out  by  the  demonstrators 
to  be  handed  round.  These  illustrate  types  of  all  the  great 
groups  of  the  plant  kingdom,  from  the  highest  (flowering)  plants. 
Ask  the  demonstrator  any  questions  that  occur  to  you  about 
them. 

Examine  also  the  fully  labelled  demonstration  series. 

C.  DIFFERENCES  BETWEEN  SPECIES  OF  THE  SAME  GENUS. 

Examine  carefully  fresh  specimens  of  two  or  three  species 
of  flowering  plant  belonging  to  the  same  genus.  Write  down  the 
differences  which  strike  you,  and  get  the  demonstrator  to  correct 
your  notes  and  to  point  out  the  characteristic  differences  in  case 
you  have  failed  to  observe  them.  [The  species  used  must  depend 
upon  the  time  of  year  and  upon  what  can  be  obtained.  In 
the  autumn  Quercus  robur,  Q.  sessiliflora  and  Q.  ilex  are  suitable  ; 
in  April  Veronica  agrcstis,  V.  hederifolia  and  V.  tournefortii ; 
in  May  or  June  Ranunculus  acer,  R.  repens  and  R.  bulbosus.] 

D.  EXCEPTIONS  TO  THE  CHARACTERISTIC  FORMS  AND  STRUCTURES 
OF  PLANTS  AND  ANIMALS. 

(Demonstration  Specimens.) 

(1)  A  Branching  Animal. — Fresh  or  museum  specimens  of  a 
hydroid  polyp  or  a  bryozoon  "  colony."     Each  individual  has  a 
mouth  surrounded  by  tentacles  and  a  gut.     It  consumes  solid 
food  consisting  of  tiny  organisms  living  in  the  sea,  and  is  therefore 
an  animal  in  spite  of  the  facts  that  it  is  fixed  to  one  spot  and 
that  the  whole  "  colony  "  has  a  branching  form  like  a  plant. 

(2)  A    Compact    Plant. — Mamillaria,    or   other   cactus    from 
subtropical   America,   shows   no   obvious   division   of  its   shoot 
into  stem  and  leaves.     It  has  a  branching  root  in  the  soil  like 
an  ordinary  plant,  but  its  shoot  consists  of  a  compact  fleshy 
green  stem  bearing  spines  or  bristles  which  represent  the  leaves. 
The  compact  form  decreases  the  evaporating  surface  and  at  the 
same  time  stores  water  which  is  held  by  the  abundant  mucilage 
the  plant  contains,  so  that  the  cactus  is  able  to  live  in  a  hot,  dry 
climate. 

(3)  An  Animal  with  no  Mouth. — The  tapeworm  is  a  parasitic 
animal  which  absorbs  its  liquid  organic  food  from  the  intestines 
of  the  animal  in  which  it  lives.     It  has  no  mouth  or  gut,  having 
lost  these  by  degeneration  in  correspondence  with  its  habit  of 
life. 


PRACTICAL   WORK  35 

(4)  A  Plant  which  consumes  Solid  Organic  Food.— The  sundew 
(Drosera)  and  the  pitcher  plant  (Nepenthes)  are  insectivorous 
plants  which  digest  and  absorb  the  products  of  insects  that  fall 
on  to  the  leaf  or  into  the  pitcher  (a  modified  part  of  the  leaf). 
This  is  a  character  not  possessed  by  most  plants,  but  it  serves 
to  demonstrate  the  common  powers  of  plant  and  animal  proto- 
plasm. In  other  respects  the  sundew  and  the  pitcher  plant  are 
ordinary  flowering  plants. 


CHAPTER   II 

ORGANIC   SUBSTANCES  AND   THEIR 
CHEMICAL  CHARACTERS 

ALL  the  activities  of  life  depend  upon  protoplasm, 
which  is  therefore  the  essential  part  of  every  living 
organism.  But  by  no  means  all  parts  of  an  organism 
consist  of  or  contain  living  protoplasm.  For  instance, 
the  heartwood  and  outer  bark  of  a  tree,  the  hairs, 
feathers,  nails  or  hoofs  of  a  warm-blooded  animal  are 
destitute  of  protoplasm.  These  lifeless  parts  of  an 
organism  are,  however,  all  formed  from  or  by  proto- 
plasm, and  they  consist  mainly  of  complex  chemical 
compounds,  containing  carbon,  hydrogen  and  oxygen, 
often  also  nitrogen  and  sulphur,  as  well  as  other  elements, 
which  are  called  "  organic  "  compounds  because  they 
are  associated  with  organisms.  There  is  no  sharp 
distinction  between  organic  and  inorganic  compounds 
in  chemistry,  and  many  of  the  simpler  organic  compounds 
formed  by  organisms  can  also  be  made  synthetically 
in  the  laboratory. 

All  the  organic  compounds  contain  the  element 
carbon,  organic  chemistry  being  sometimes  known  as 
"  the  chemistry  of  the  carbon  compounds,"  though 
many  substances  containing  carbon  (for  instance,  cal- 
cium carbonate)  are  not  specially  associated  with 
organisms.  Of  the  immense  number  of  organic  com- 
pounds known  certain  classes  are  specially  important 
in  the  structure  and  activities  of  living  organisms,  and 


WATER.      CARBOHYDRATES  37 

we  must  have  some  knowledge  of  the  nature  and  proper- 
ties of  these  before  we  can  understand  the  nature  and 
functions  of  protoplasm. 

We  must,  of  course,  be  careful  to  distinguish  between 
an  organic  substance  which  is  a  mixture  of  chemical 
compounds  in  various  proportions,  and  a  chemical 
compound  which  has  a  perfectly  definite  chemical 
composition,  i.e.  its  molecule  or  ultimate  unit  of 
structure  consists  of  a  definite  number  of  atoms  of 
different  elements  arranged  in  a  definite  way.  Milk, 
for  instance,  is  an  organic  substance  which  has  a 
varying  composition,  being  a  mixture  in  various  propor- 
tions of  the  chemical  compounds  water,  milk  sugar, 
various  definite  fats,  proteins,  salts,  etc.  Such  organic 
substances  as  wood,  horn,  hair,  etc.,  are,  similarly, 
mixtures  of  chemical  compounds. 

Of  the  chemical  compounds  which  enter  into  the 
composition  of  the  body  of  an  organism,  water  (H2O) 
is  of  first  importance,  as  we  shall  see  in  detail 
later.  It  is  essential  to  the  structure  of  living  proto- 
plasm, and  is  found  in  greater  or  less  proportion  in 
all  parts  of  the  body.  More  than  90  per  cent,  of  the 
weight  of  a  herbaceous  plant  is  water,  as  can  easily  be 
shown  by  weighing  the  plant  and  then  heating  it  at 
100°  C.  till  it  loses  no  more  water  and  weighing  again. 

The  three  classes  of  chemical  compounds  which  play 
the  leading  part  in  organisms  are  the  carbohydrates, 
the  fats,  and  the  proteins,  the  two  former  consisting 
of  carbon,  hydrogen  and  oxygen,  the  latter  with  nitrogen, 
sulphur,  and  sometimes  phosphorus  in  addition. 

THE  CARBOHYDRATES. 

These  are  a  class  of  carbon  compounds  in  whose 
molecules  the  atoms  of  hydrogen  and  oxygen  are 


38       ORGANIC   SUBSTANCES  I    CHEMICAL   CHARACTERS 

generally   present  in   the  proportion   of   two   to   one, 
as  in  water. 

The  Sugars. 

The  most  important  carbohydrates  are  the  sugars,  of 
which  there  are  many  different  kinds.  Glucose  (grape 
sugar),  C6H12O6,  Icevulose,  C6H12O6,  sucrose  (cane  sugar), 
C12H22On,  and  maltose,  C12H22On,  are  the  only  ones 
we  need  mention  here.  It  will  be  seen  that  the  two 
former  have  the  same  number  of  atoms  in  the  molecule, 
and  so  have  the  two  latter.  They  differ,  however,  in 
the  way  in  which  the  atoms  are  grouped,  and  this 
leads  to  differences  in  crystalline  form,  and  to  differences 
in  reaction  with  other  compounds. 

The  sugars  are  readily  soluble  in  water ;  moreover,  they 
easily  move  about  the  body  by  liquid  diffusion,  and  their 
molecules  possess  a  large  amount  of  potential  (chemical) 
energy,  so  that  when  the  molecule  is  oxidised  and  broken 
up  a  great  deal  of  energy  is  set  free  in  the  kinetic  form. 
The  kinetic  energy  which  appears  in  organisms  as  heat  and 
movement  is  mainly  derived  from  the  oxidation  and  breaking 
up  of  sugars.  For  these  reasons  the  sugars  are  extremely 
important  in  the  sum  total  of  the  chemical  changes, 
called  metabolism,  which  are  perpetually  occurring  in 
the  living  protoplasm  of  plants  and  animals. 

Glucose  (grape  sugar),  C6H12O6,  is  the  most  prominent 
of  the  sugars  in  the  economy  of  organisms.  While 
glucose  is  a  relatively  stable  substance  in  the  test  tube, 
it  takes  up  oxygen  more  easily  than  other  sugars,  and 
thus  acts  as  a  moderately  strong  "  reducing  agent," 
i.e.  it  can  take  oxygen  from  (or  "  reduce  ")  certain  other 
substances.  An  example  of  this  action  is  the  behaviour 
of  glucose  solution  when  mixed  with  a  cupric  salt 
(i.e.  a  salt  of  copper  containing  much  oxygen)  in  alkaline 


SUGARS  39 

solution  —  such  as  Fehling's  solution  —  and  warmed. 
Some  of  the  oxygen  is  taken  from  the  cupric  salt,  and 
red  cuprous  oxide  is  formed.  This  is  the  ordinary  test 
for  sugars  that  behave  in  this  way  —  the  "  reducing 
sugars  "  as  they  are  called.  Maltose  is  also  a  reducing 
sugar,  though  to  a  less  extent  than  glucose.  On  the 
other  hand,  sucrose  (cane  sugar)  does  not  reduce 
Fehling's  solution  at  all. 

In  the  living  cell  glucose  very  readily  takes  up  oxygen 
and  breaks  up  its  molecule  into  carbon  dioxide  and 
water.  This  is  the  basis  of  the  process  of  respiration, 
which  is  of  fundamental  importance  in  the  economy  of 
living  organisms  :  — 

C6H12O6  +  6O2  =  6CO,  +  6H2O 

glucose         oxygen      carbon         water 


Sucrose    or   saccharose   (cane  sugar),   C12H22On,  is 

the  most  important  sugar  commercially.  It  is  the 
main  product  of  the  sugar  cane  and  the  sugar  beet, 
and  is  the  principal  sugar  we  use  as  food.  It  is  very 
much  sweeter  than  glucose.  The  importance  of  sugar 
as  a  food  for  animals  depends  on  the  characters  already 
mentioned,  solubility  and  high  potential  energy.  It  is 
thus  easily  absorbed  through  the  wall  of  the  alimentary 
canal  into  the  blood,  and  the  readiness  with  which  it 
is  oxidised  places  large  amounts  of  energy  at  the  dis- 
posal of  the  muscles,  where  the  energy  appears  in  the 
kinetic  form,  as  motion  and  heat.  Sugar  is,  in  fact, 
used  in  this  way  by  all  living  cells,  but  most  energetically 
by  the  muscles,  which  are  the  great  energy  spenders 
of  the  animal  body. 

Sucrose  is  not  a  reducing  sugar,  and,  in  the  living 
cell,  is  converted  into  glucose  and  laevulose  before  being 
oxidised.  This  process  of  the  conversion  of  sucrose 


40       ORGANIC   SUBSTANCES  I    CHEMICAL  CHARACTERS 

into  glucose  and  laevulose  is  an  example  of  hydrolysis, 
i.e.  the  splitting  of  a  molecule  by  reaction  with  water. 

C12H22On  +  H20  =  C6H]206  +  C6H1206 

sucrose  water  glucose  laevulose 

Various  sugars  are  found  mixed  in  different  proportions 
in  living  cells  and  are  constantly  being  converted  from 
one  form  into  another. 

The  Polysaccharides. 

These  are  another  important  class  of  carbohydrates. 
They  derive  their  name  from  the  fact  that  they  are 
formed  by  the  putting  together  of  many  molecules  of 
sugar  to  form  one  molecule  of  polysaccharide,  a  process 
known  as  condensation.  The  most  important  poly- 
saccharides  of  plants  are  the  starches  and  the  celluloses. 
There  are  many  different  kinds  of  each,  but  for  our 
purpose  we  may  treat  them  in  each  case  as  one. 

Starch  is  formed  as  solid  grains  in  the  protoplasm 
of  the  living  cells  of  plants.  The  substance  of  each 
grain  is  laid  down  in  concentric  layers  round  a  centre 
called  the  hilum  of  the  starch  grain.  Starch  is  formed 
by  the  union  of  many  molecules  of  glucose  with  the 
elimination  of  one  molecule  of  water  from  each  : — 

«C6H1206  =  (C6H1005)n  +  nH20 

The  value  of  n  has  not  been  exactly  determined,  but 
it  is  probably  somewhere  about  100.  Thus  the  starch 
molecule  is  far  more  complex  than  the  glucose  mole- 
cule. Starch  has  a  very  characteristic  blue  reaction 
with  watery  solution  of  iodine  in  potassium  iodide. 
Most  starches  are  quite  insoluble  in  cold  water  ;  they 
are  hydrolysed  in  the  presence  of  dilute  mineral  acids 
into  various  simpler  carbohydrates,  and  eventually 


POLYSACCHARIDES.      FATS  4! 

into  glucose.  Hydrolysis  of  starch  also  takes  place  in 
plant  cells  (see  below).  Starch  is  a  very  common  and 
widespread  substance  in  plant  cells.  It  bulks  largely 
in  the  economy  of  plants,  though  it  stands  second  to 
the  sugars  in  this  respect.  It  also  forms  a  very  valu- 
able food  for  animals,  being  hydrolysed  into  sugars 
(or  rather  its  hydrolysis  is  catalysed)  by  the  action  of 
the  digestive  juices  of  the  mouth  and  of  the  small 
intestine. 

Cellulose  is  a  substance  of  even  more  complex  composi- 
tion than  starch.  Its  empirical  formula  is  the  same, 
i.e.  it  has  the  same  proportion  of  carbon,  hydrogen  and 
oxygen  atoms  in  the  molecule,  though  a  much  higher  total 
number.  Thus,  if  starch  be  represented  by  (C6H10O5)n 
we  may  represent  cellulose  by  (C6H10O5)m. 

Cellulose  is  of  immense  importance  because  it  forms 
the  main  substance  of  the  cell  walls  of  most  plants, 
and  thus  the  "  skeleton  "  of  the  plant  body.  Linen 
and  cotton  are  nearly  pure  cellulose,  and  paper  contains 
a  varying  amount  according  to  the  plant  substance 
from  which  it  is  made.  Cellulose  is  formed  in  a  similar 
manner  to  starch  by  the  condensation  of  many  mole- 
cules of  sugar.  It  is  not  coloured  blue  with  iodine, 
but  it  can  be  altered,  e.g.  by  the  action  of  strong 
sulphuric  acid  into  a  substance  which  resembles  starch 
in  so  far  as  it  is  so  coloured  with  iodine. 

THE  FATS. 

These,  like  the  carbohydrates,  consist  of  carbon, 
hydrogen  and  oxygen,  but  the  fat  molecule  is  much 
larger  than  the  sugar  molecule,  and  contains  a  much 
smaller  proportion  of  oxygen.  Fats  are  widely  dis- 
tributed in  plant  cells,  and  form  a  characteristic  store 
of  non-nitrogenous  food  in  many  seeds,  e.g.  linseed, 


42   ORGANIC  SUBSTANCES  :  CHEMICAL  CHARACTERS 

cottonseed,  coconut,  castor  oil  seed.  They  contain  an 
even  larger  store  of  potential  energy  per  molecule  than 
the  carbohydrates,  and  thus  require  more  oxygen  for 
complete  oxidation.  Of  numerous  kinds  two  examples 
are  palmitin  (C51H98O6)  and  olein  (C57H104O6).  Fats 
occur  in  plant  cells  as  complex  mixtures  of  different 
individual  fats.  They  are  usually  found  in  the  liquid 
state,  and  are  then  often  spoken  of  as  "  oils." 

Fats  are  even  more  widespread  and  important  in  the 
animal  than  in  the  plant  body,  forming  a  very  valuable 
reserve  store  of  energy. 

THE  PROTEINS. 

These  are  the  most  important  of  the  nitrogen- 
containing  organic  substances.  The  molecule  consists 
of  carbon,  hydrogen,  oxygen,  nitrogen  and  sulphur, 
with  sometimes  phosphorus  in  addition.  The  protein 
molecules  are  the  largest  and  most  complex  of  any 
chemical  compounds,  each  molecule  consisting  of  many 
hundreds,  or  even  thousands,  of  atoms.  The  different 
groups  of  atoms  composing  these  huge  molecules  have, 
like  the  atoms  composing  the  molecules  of  all  chemical 
compounds,  a  perfectly  definite  position  in  regard  to 
one  another.  The  more  numerous  the  atoms  the  greater 
will  be  the  number  of  different  relative  positions  possi- 
ble. Hence  there  are  an  enormous  number  of  distinct 
protein  molecules  with  comparatively  small  differences 
between  them. 

Now  protein  molecules  form  the  essential  basis 
of  protoplasm.  Protoplasm,  as  we  shall  see  in 
succeeding  chapters,  has  the  same  general  character- 
istics and  powers  in  all  living  organisms,  but  it  is 
obvious  that  the  protoplasm  of  every  species  differs 
in  some  way  from  that  of  every  other  species,  or  it 


PROTEINS  43 

could  not  build  up  a  different  body.  It  is  probable 
that  differences  between  species  ultimately  depend 
upon  comparatively  slight  differences  in  structure 
between  some  of  the  proteins  present,  or  between  the 
relative  amounts  of  the  different  proteins  present,  or 
perhaps  between  the  ways  in  which  different  protein 
molecules  are  built  up  into  more  complex  protoplasmic 
structures,  of  which  we  actually  know  practically 
nothing.  Such  biochemical  differences  between  even 
closely  allied  species  have  recently  been  shown  to  hold 
for  other  important  organic  substances  of  the  body, 
for  instance  between  the  starches  of  different  species 
of  plants  and  between  the  haemoglobins  (the  red  pig- 
ment of  the  blood)  of  different  species  of  animals.  It 
has  been  shown  that  the  starch  of  every  species  (out 
of  many  hundreds  examined)  is  in  some  respect  different 
from  the  starch  of  every  other  species  ;  and  the  same 
is  true  of  haemoglobin .  The  proteins  are  much  more 
difficult  to  investigate,  but  when  we  consider  that  the 
molecules  of  many  of  them  are  much  more  complicated, 
it  becomes  obvious  that  the  possibilities  of  slight  but 
perfectly  definite  differences  are  much  more  numerous. 

Proteins  as  a  class  show  certain  colour  changes 
when  acted  upon  by  certain  substances  ;  for  instance 
a  yellow  colour,  when  after  boiling  with  nitric  acid 
ammonia  is  added  ;  a  violet  colour,  with  a  drop  of 
dilute  copper  sulphate  followed  by  excess  of  caustic 
soda.  A  dilute  solution  of  iodine  in  potassium  iodide, 
which  colours  starch  blue,  stains  proteins  yellow.  In 
such  ways  proteins,  as  a  class,  can  be  distinguished 
from  other  classes  of  organic  compounds.  Into  the 
characters  of  the  different  proteins  we  shall  not  attempt 
to  enter  here. 

Apart  from  their  universal  occurrence  in  living  proto- 


44       ORGANIC  SUBSTANCES  :    CHEMICAL  CHARACTERS 

plasm,  as  the  essential  part  of  its  structure,  proteins 
occur  in  more  or  less  solid  aggregations  as  grains  (called 
"  aleurone  grains ")  of  non-living  substance  in  the 
cells  of  certain  parts  of  plants,  such  as  seeds.  Here 
they  represent  a  store  of  protein  substance  used  as 
food  by  the  young  seedling,  and  are  comparable  in 
this  respect  with  starch  grains  or  oil  (fat)  globules, 
which  similarly  form  stores  of  non-nitrogenous  sub- 
stance. 

There  are  of  course  many  other  classes  of  organic 
substances  besides  the  three  mentioned.  In  plants,  for 
instance,  the  so-called  "  aromatic  substances  "  occur 
universally.  They  contain  carbon,  hydrogen  and 
oxygen,  and  are  founded  on  the  structure  known  as 
the  benzene  ring,  or  a  similar  ring,  but  our  knowledge 
of  their  role  in  the  economy  of  plants  is  still  obscure. 
The  three  classes  of  organic  substances — the  carbo- 
hydrates, fats  and  proteins — briefly  described  comprise 
the  most  inportant  organic  substances  entering  into 
the  composition  of  organisms,  and  they  can  all  be 
used  as  foods  by  animals. 

Enzymes. — These  are  a  very  important  class  of 
substances,  whose  exact  chemical  composition  is  still 
unknown,  but  many  of  them  can  easily  be  isolated. 
Their  importance  consists  in  the  fact  that  they  act  as 
catalysts  to  many  of  the  chemical  changes  occurring 
in  living  protoplasm.  A  catalyst  is  a  substance  that ' 
facilitates  a  chemical  change  without  itself  entering 
into  the  final  products  of  the  change.  It  probably 
works  by  temporarily  entering  into  combination  with 
the  substance  or  substances  in  reaction,  causing  a 
disruption  of  their  molecules,  and  ultimately  separating 
from  the  products.  Owing  to  the  fact  that  the  enzyme 
itself  is  not  involved  in  the  final  product,  a  very  small 


ENZYMES  45 

amount  of  enzyme  is  sufficient  to  carry  out  the  change 
in  large  quantities  of  substance. 

Enzymes  in  great  variety  exist  in  the  living  body, 
each  kind  acting  upon  a  particular  class  of  substance. 
Thus  the  Upases l  facilitate  the  hydrolysis  of  fats, 
which  are  decomposed  into  simpler  substances  ;  the 
proteases  have  a  similar  action  upon  proteins.  Of  the 
enzymes  which  work,  upon  carbohydrates,  diastase, 
which  converts  starch  into  sugar,  is  the  best  known. 
The  first  stage  of  the  reaction  produces  dextrin  (a 
soluble  polysaccharide  of  the  same  empirical  formula 
as  starch)  and  maltose  : — 

(C6H1005)n  +  H20-->(C6H1005),  +  C12H220U 

starch  water  dextrin  maltose 

The  molecule  of  maltose  is  hydrolysed  (its  hydrolysis 
being  catalysed  by  another  enzyme,  maltase)  into  two 
molecules  of  glucose  : — 

C12H22On  +  H20  =  2C6H1206 

maltose  water  glucose 

Invertase  similarly  catalyses  the  hydrolysis  of  sucrose 
into  one  molecule  of  glucose  and  one  molecule  of  the 
closely  allied  sugar  Icevulose  :  — 

C12H,2On  +  H20  =  C6H]206  +  C6H1206 

sucrose  water  glucose  laevulose 

Cytases  facilitate  the  hydrolysis  of  the  celluloses  into 
simpler  soluble  carbohydrates. 

.  Many,  if  not  all,  of  the  enzymes  have  what  is  called 
a  reversible  action,  i.e.  they  may  bring  about  a  change 
in  either  direction,  e.g.  of  starch  into  sugar,  or  of  sugar 
into  starch.  The  direction  in  which  the  reaction  actually 
proceeds  at  any  given  moment  depends  upon  the  rela- 

1  The  uniform  termination  -ase  is  used  in  the  naming  of  enzymes. 


46       ORGANIC   SUBSTANCES  I    CHEMICAL   CHARACTERS 

tive  concentration  of  the  soluble  end  products.  Thus, 
if  the  sugar  concentration  is  low,  starch  will  be  converted 
into  sugar,  but  when  it  rises  above  a  certain  point  the 
sugar  will  be  converted  into  starch.  We  shall  see  in 
the  sequel  that  these  reversible  reactions  are  very 
important  in  the  living  organism. 

In  nature  organic  compounds  always  occur  as 
constituents  or  products  of  living  organisms,  but 
chemists  can  now  make  many  of  them  out  of  simpler 
substances  in  the  laboratory,  for  instance  the  sugars, 
and  even  some  of  the  simpler  proteins. 

PRACTICAL  WORK. 
CARBOHYDRATES,  FATS  AND  PROTEINS. 

(1)  Compare  the  samples  of  solid  glucose  and  of  solid  sucrose 
provided.     Note  that  the  first  is  a  crystalline  powder,  while  the 
second  forms  large  well-defined  crystals.     Compare  their  sweet- 
ness  by   tasting.     Dissolve   the   samples   in   two   separate   test 
tubes  each  containing  a  little  water,  and  note  that  each  dissolves 
completely.     Test  a  little  of   each  with  very  dilute  solution  of 
iodine  in  potassium  iodide  x  and  note  that  there  is  no  coloration. 
Add  a  few  drops  of  Fehling's  solution  (copper  sulphate  made 
strongly  alkaline)  to  each  test  tube,  and  heat.     Note  that  one 
solution  turns  red  owing  to  the  formation  of  red  cuprous  oxide, 
while  the  other  does  not. 

(2)  Examine  a  thin  slice  of  potato  under  the  low  and  high  powers 
of  the  microscope.     Draw  one  or  two  of  the  starch  grains  under 
the  high  power.     Measure  their  long  and  short  diameters  with 
the  micrometer  eyepiece  and  express  the  values  in  n  (see  Practical 
Work  I  (4)  p.  33).     Treat  the  section  with  very  dilute  iodine 
solution  and  examine  again. 

(3)  The    tube    provided    contains    very    dilute    starch    paste. 
Pour  a  little  into  a  watchglass,  and  add  one  drop  of  very  dilute 
iodine  solution.     Now  add  a  little  taka-diastase  (a  commercial 
preparation  of  diastase  also  containing  maltase,  see  p.  45)  to  the 
contents  of  the  tube,  and  shake  the  tube  vigorously  at  intervals. 
After  a  minute  pour  a  little  of  the  mixture  into  another  watch- 

1  This  solution  will  be  alluded  to  in  future  simply  as  "  iodine 
solution." 


PRACTICAL   WORK  47 

glass  and  add  a  drop  of  very  dilute  iodine.  Shake  the  tube 
vigorously  again  at  intervals  and  after  another  two  minutes 
pour  a  little  into  a  third  watchglass,  adding  a  drop  of  very  dilute 
iodine.  Note  the  colours  in  the  three  cases.  The  pinkish  colour 
with  iodine  is  due  to  the  formation  of  dextrin,  a  carbohydrate 
intermediate  between  starch  and  sugar.  On  adding  a  few  drops 
of  Fehling's  solution  to  the  contents  of  the  test  tube  after  half 
an  hour,  and  heating,  a  slight  reducing  effect  should  be  obtained. 
This  is  due  to  the  glucose  present. 

(4)  Test  with  iodine  solution  a  piece  of  good  writing  paper 
(nearly  pure  cellulose)   across  which  strong  sulphuric  acid  has 
been  streaked. 

(5)  Examine  the  demonstration  specimen  of  paper  boiled  in 
strong  sulphuric  acid,  neutralized  with  alkali  and  then  treated  with 
Fehling's  solution  to  demonstrate  the  formation  of  a  reducing 
sugar   (glucose)  ;    also  of  the  cut  date  stones   (the  very  thick 
walls  of  whose  cells  consist  of  cellulose)  similarly  treated. 

(6)  Note    that    desiccated    coconut    "  meat "    (food    material 
used  by  the  embryo  growing  into  a  seedling)  is  oily  to  the  touch. 
Place  some  in  a  test  tube  with  water  and  boil :    the  oil  rises  to 
the  surface.     Add  a  few  drops  of  Sudan  3  (an  aniline  dye)  and 
allow  to  stand  :    the  oil  is  coloured  red.     Add  a  few  drops  of 
Sudan  3  to  olive  oil  in  a  test  tube  :  the  oil  is  coloured  red.     Pour 
half  the  red  oil  into  another  test  tube.     Add  a  little  alcohol  to 
one  and  a  little  water  to  the  other.     Note  the  position  taken  up 
by  the  water  and  alcohol  respectively  in  relation  to  the  oil : 
also  the  coloration  resulting. 

(7)  The  two  substances  provided  in  separate  test  tubes  are 

(a)  white  of  egg  shaken  up  with  water,  which  is  a  typical  protein, 

(b)  bean  meal  shaken  up  with  salt  solution.     Divide  each  into 
three  portions  and  subject  each  set  of  three  to  the  following 
tests,  noting  the  coloration  in  each  case : — 

(i)  Add  a  few  drops  of  strong  nitric  acid.  Boil  and  then 
cool  thoroughly.  Add  a  few  drops  of  ammonia  solution)  xantho- 
proteic  reaction). 

(ii)  Add  one  drop  of  dilute  copper  sulphate  solution.  Now 
add  excess  of  caustic  soda  or  potash  (biuret  reaction). 

(iii)  Add  a  few  drops  of  iodine  solution. 


CHAPTER   III 

SOME   PHYSICAL  CHARACTERS   OF  ORGANIC 
SUBSTANCES 

Crystalloids  and  Colloids. — A  solid  substance,  when 
placed  in  water,  may  behave  in  one  of  several  ways. 
First,  it  may  react  chemically  with  the  water,  as  for 
instance  sodium,  which  breaks  up  the  molecule  of 
water,  combining  with  the  oxygen  and  liberating 
hydrogen.  Secondly,  it  may  dissolve  in  the  water  to 
form  a  true  solution,  as  for  instance  common  salt 
(sodium  chloride),  the  salt  seeming  to  disappear  in 
the  solution  (where  it  is  called  the  solute,  the  dis- 
solving substance  being  the  solvent}.  Thirdly,  it  may 
remain  practically  unaltered,  as  for  instance  quartz 
sand  (silica).  If,  however,  such  a  substance  as  the 
coarse  grains  of  silica  are  ground  in  a  very  finely 
grinding  mill  into  finer  and  finer  particles,  a  point  of 
fineness  is  ultimately  reached  at  which  the  particles 
stay  suspended  in  the  water,  without  settling,  to 
form  what  is  known  as  a  colloid  *  solution,  or  sol,  which 
differs  from  a  true  solution  in  several  important 
respects.  This  fourth  kind  of  behaviour  with  water 
is  a  very  characteristic  feature  of  many  organic 
compounds. 

Broadly  speaking,  the  substances  which  form  true 
solutions  with  water,  or  some  other  liquid  solvent, 
have  small  or  comparatively  small  molecules,  such  for 
instance  as  simple  inorganic  salts,  and  many  of  the 

1  Greek  KoAXa,  glue,  from  the  jelly-like  nature  of  the  most  familiar 
colloids  (gels).  See  p.  53. 

48 


CRYSTALLOIDS   AND   COLLOIDS  49 

simpler  organic  compounds,  such  as  the  sugars.  On 
evaporation  of  the  solvent  they  reappear,  often  in  the 
form  of  crystals.  Substances  which  behave  in  this 
way  are  known  as  crystalloids.  The  substances  which 
when  mixed  with  water  become  dispersed  in  very  fine 
particles  through  the  water,  each  particle  consisting  of 
a  number  of  molecules  aggregated  together,  are  called 
colloids.  It  is  to  be  noted  that  many  simple  substances 
— even  elements  such  as  gold  and  platinum — may  be 
prepared  in  such  a  form  that  they  behave  as  colloids 
with  water,  i.e.  they  become  dispersed  in  minute  particles 
through  the  liquid.  We  now  know  that  we  cannot  sharply 
divide  substances  into  the  two  categories,  crystalloids 
and  colloids,  but  speak  of  a  substance,  in  a  given 
condition,  behaving  as  "  a  crystalloid  or  as  a  colloid  " 
with  water,  or  some  other  solvent.  Nevertheless  the 
organic  compounds  with  large  molecules,  e.g.  poly- 
saccharides,  proteins,  etc.,  commonly  do  behave  as, 
and  are  therefore  commonly  spoken  of  as,  colloids. 

The  aggregate  of  the  dispersed  particles  of  a  colloid 
is  known  as  the  disperse  phase,  the  continuous  liquid 
medium  in  which  they  are  dispersed  as  the  continuous 
phase  of  the  colloid  sol.  The  disperse  phase  does  not 
necessarily  consist  of  solid  particles,  it  may  consist  of 
very  minute  liquid  droplets,  and  then  the  sol  is  called 
an  emulsion  sol,  or  an  emulsoid,  while  if  the  disperse 
phase  is  solid  we  have  a  suspension  sol,  or  a  suspensoid. 

Coarse  particles  or  droplets,  suspended  in  a  liquid, 
form  ordinary  suspensions  or  emulsions,  and  tend  to 
sink  slowly  to  the  bottom,  or  rise  to  the  top,  as  they 
are  heavier  or  lighter  than  the  liquid.  Thus  the  fat 
globules  suspended  in  milk  slowly  rise  to  the  surface 
as  cream.  If  we  shake  up  a  handful  of  soil  in  a  beaker 
of  water,  the  coarser  and  heavier  particles  sink  to  the 
4 


50      PHYSICAL   CHARACTERS   OF   ORGANIC   SUBSTANCES 

bottom  at  once,  others  sink  more  slowly,  but  the  very 
fine  colloidal  particles  (clay)  remain  suspended  indefi- 
nitely, forming  a  colloid  sol  with  the  water. 

Microscopic  and  Ultramicroscopic  Particles.— The 
larger  of  these  very  fine  colloid  particles,  such  as  those 
of  an  Indian  ink  sol,  may  be  clearly  seen  under  the 
high  power  of  the  microscope.  When  they  are  quite 
freely  suspended,  that  is  not  adhering  to  the  glass 
slide  or  coverslip,  they  may  be  seen  to  be  in  continuous 
oscillation,  or  intermittent  jerky  movement.  This  is 
due  to  the  continuous  bombardment  of  the  disperse 
particles  by  the  molecules  of  the  liquid,  which  are  of 
course  themselves  in  constant  motion.  At  any  given 
moment  the  probability  is  that  this  bombardment  will 
be  unequal  on  different  sides  of  the  suspended  particle, 
and  this  will  therefore  move  away  from  the  side  on 
which  it  is  most  heavily  bombarded.  If  the  bombard- 
ment happens  to  be  momentarily  equal  on  all  sides 
the  particle  will  remain  motionless.  This  kind  of 
movement  is  called  Brownian  movement,  from  its  dis- 
coverer, the  botanist  Robert  Brown. 

The  disperse  particles  of  the  finer  sols,  such  for 
instance  as  a  Congo  Red  sol,  are  too  small  to  be  seen 
under  the  highest  powers  of  the  microscope,  and  the 
sol  appears  clear.  With  the  highest  powers  and  the 
best  definition  the  smallest  particles  that  can  be  clearly 
defined  under  the  microscope  are  those  not  less  than 
about  '15  p I  —  '00015  mm.  in  diameter.  But  by 
means  of  an  instrument  called  the  ultramicroscope — 
which  consists  essentially  of  a  high  power  microscope 
with  lateral  instead  of  vertical  illumination — when 
lighting  from  the  side  and  viewing  against  a  dark 

1  fi  is  one-thousandth  part  of  a  millimeter  and  is  the  ordinary  unit 
of  microscopic  measurement. 


ULTRAMICROSCOPIC   BODIES  51 

background,  smaller  objects  than  these  can  be  detected, 
though  they  cannot  be  clearly  defined.  They  are  seen 
as  tiny  blurs  of  light  owing  to  diffraction  of  the  rays 
round  the  ultramicroscopic  particles.  By  this  method 
particles  of  about  '005  p  (=  5  /XJLI)  x  may  be  detected, 
which  we  know  from  calculations  based  on  other  data 
is  about  the  size  of  the  very  large  molecules  of  soluble 
starch.2  The  disperse  particles  of  very  fine  sols  run 
down  to  perhaps  2 '5  /LI/M,  the  size  of  the  molecules  of 
haemoglobin,  the  red  pigment  of  the  blood.  The 
molecules  and  ions  (single  atoms,  or  small  groups  of 
atoms,  into  which  a  fine  solute  may  be  dissociated) 
of  true  (crystalloid)  solutions  are  much  smaller  than 
this,  from  '5/^t  (alcohol  molecule)  down  to  perhaps 
•i  \L\L,  which  is  about  the  diameter  of  a  hydrogen  ion 
(separate  atom  of  hydrogen). 

To  give  some  more  easily  comprehensible  notion  of 
the  relative  sizes  of  these  exceedingly  minute  objects  : 
if  we  imagine  the  hydrogen  ion  magnified  a  million 
times,  so  that  it  was  just  visible  to  the  naked  eye  as 
the  smallest  possible  ink  dot  on  a  sheet  of  paper,  then 
a  very  large  organic  molecule  (such  as  might  just  be 
detected  with  the  ultramicroscope),  if  it  were  magnified 
to  the  same  extent,  would  be  about  the  size  of  a  small 
pea,  and  a  very  small  bacterium  (the  smallest  organisms 
that  are  just  visible  with  the  microscope)  would  be 
twice  the  diameter  of  a  football.  Represented  on  this 
scale  a  real  pea  would  appear  more  than  three  miles 
in  diameter. 

Internal  Surface  of  Colloids.—The  very  large  number 
of  particles  or  droplets  forming  the  disperse  phase  of 

1  fifj,  is  one-thousandth  of  i  ft,  and  is  the  unit  of  ultramicroscopic 
measurement. 

1  Soluble  starch  is  a  simpler  form  of  starch  produced  on  boiling 
with  dilute  hydrochloric  acid  and  forming  a  colloid  sol  with  water. 


52      PHYSICAL   CHARACTERS   OF  ORGANIC   SUBSTANCES 

a  colloid  soil  present  collectively  an  enormous  surface. 
If  we  take  a  solid  spherical  object,  such  as  an  orange, 
and  cut  it  into  halves,  it  is  clear  that  we  have  greatly 
increased  the  exposed  surface,  for  to  the  original  external 
surface  we  have  added  the  cut  surfaces  of  the  two 
halves.  If  we  now  cut  the  halves  into  quarters  the 
total  surface  is  still  further  increased,  and  the  more 
we  divide  the  orange  the  greater  the  surface  exposed. 
Thus  the  collective  surfaces  of  all  the  disperse  particles 
in  a  drop  of  a  colloid  sol  are  immensely  greater  than 
the  surface  of  the  sphere  which  would  be  formed  if 
all  the  particles  in  the  drop  were  aggregated  in  a 
single  mass. 

Surface  Energy,  Surface  Tension  and  Adsorption.— 
Now,  where  a  surface  exists,  i.e.  where  solid  or  liquid 
matter  is  in  contact  with  other  matter,  that  surface 
is  the  seat  of  free  energy,  because  of  the  break  in 
action  of  molecular  forces  at  the  surface,  and  this 
is  expressed  in  what  is  called  surface  tension.  The 
surface  of  a  drop  of  liquid  in  air,  for  instance,  tends 
to  contract,  and  that  is  why  a  free  drop  of  liquid  assumes 
the  spherical  shape,  in  which  the  surface  has  contracted 
as  much  as  possible,  and  has  the  smallest  possible 
area  in  relation  to  the  mass  of  the  liquid.  Liquids 
with  a  high  surface  tension  form  drops  which  do  not 
easily  spread  upon  a  solid  surface,  because  their  high 
surface  tension  tends  to  keep  the  drop  spherical  and 
prevent  it  spreading.  Mercury,  water  and  alcohol 
form  a  series  of  liquids  with  decreasing  surface  tension, 
and  a  corresponding  increasing  readiness  to  "  wet," 
i.e.  spread  upon,  a  clean  sheet  of  glass. 

The  enormous  collective  surface,  i.e.  the  sum  of  all 
the  surfaces,  of  the  disperse  particles  or  droplets  of 
a  sol  involves  a  corresponding  amount  of  surface  energy 


ADSORPTION.      GEL   FORMATION  53 

of  the  particles  or  droplets.  This  is  of  very  great 
importance  in  determining  the  behaviour  of  the  colloid 
towards  liquids,  solutes  and  other  sols,  which  may  be 
attracted  to  the  surfaces  of  the  disperse  particles  or 
droplets,  and  held  there  by  the  surface  energy,  at  the 
same  time  decreasing  the  surface  tension  of  the  dis- 
perse phase.  This  process  is  called  adsorption.  The 
living  protoplasm  of  the  cell  is  believed  to  be  a  colloid 
sol  of  which  the  continuous  phase  is  a  solution  of  various 
crystalloids,  and  the  disperse  phase  consists  of  protein 
and  fat  droplets  or  particles.  These  take  up  and  hold 
by  adsorption  the  molecules  and  ions  of  solutes  which 
can  diffuse  into  the  cell  and  come  within  the  range 
of  their  surface  energy.  In  this  way  large  quantities 
of  various  substances  may  be  taken  in  and  held  in  the 
living  cell.  Dyes  are  taken  up  in  the  same  way  by 
protoplasm  and  by  the  other  organic  colloids  of  the 
plant  or  animal,  and  on  this  process  largely  depends 
the  staining  of  tissues  employed  in  making  microscopic 
preparations. 

Gel  Formation. — When  hydrosols  (sols  with  water 
as  the  continuous  phase)  lose  water,  for  instance  by 
evaporation,  their  disperse  particles  get  closer  and 
closer  together.  In  the  case  of  certain  sols,  including 
those  whose  disperse  phases  consist  of  large  organic 
molecules,  such  as  those  of  gelatine  for  example,  the 
molecules  or  aggregates  of  molecules  tend,  on  loss  of 
water,  to  run  together  in  chains.  As  more  water  is 
lost,  we  picture  these  chains  becoming  more  and  more 
entangled  with  one  another,  the  sol  becomes  more  and 
more  viscous  (treacle-like),  till  ultimately  a  jelly — 
technically  called  a  gel — is  formed.J  On  further  loss 
of  water  the  gel  shrinks,  and  eventually  becomes  solid. 
If  water  is  again  added  it  is  absorbed,  and  the  substance 


54  PHYSICAL  CHARACTERS  OF  ORGANIC  SUBSTANCES 

gradually  swells  till  it  passes  back  into  the  condition 
of  a  sol. 

A  gel  which  can  thus  pass  back  into  a  sol  is  called 
a  reversible  gel.  A  piece  of  solid  gelatine,  for  instance, 
will  absorb  water  till  it  passes  into  the  gel,  and  finally 
into  the  sol  condition,  in  which  the  original  gelatine  has 
disappeared  from  sight.  Allowed  to  evaporate  the  sol 
will  lose  water  till  the  gel  reappears  and  the  gelatine 
finally  becomes  solid  again  ;  and  these  opposite  pro- 
cesses can  be  carried  out  an  indefinite  number  of 
times. 

Many  organic  colloids  of  the  living  body  are  in  the 
gel  condition,  e.g.  cell  walls,  starch  grains,  gums, 
mucilages,  etc.,  and  some  of  these,  by  the  absorption 
and  loss  of  water,  can  pass  into  sols  and  back  again. 
Protoplasm  itself  may  exist  either  as  a  sol  or  as  a  gel. 
The  internal  surface  of  a  gel,  though  less  than  that  of 
the  disperse  phase  of  a  sol,  is  nevertheless  very  great, 
and  gels  accordingly  show  those  characters,  such  as 
the  power  of  adsorption,  which  depends  upon  internal 
surface. 

Membrane  Formation.— Gels  may  be  formed  from 
sols  in  other  ways  than  by  loss  of  water.  For  instance, 
when  a  sol  is  in  contact  with  some  other  substance, 
a  film  or  membrane  of  gel  structure  is  often  formed 
on  the  plane  of  contact.  The  protoplasmic  sol  always 
forms  a  membrane  or  film  on  its  free  surface,  and  these 
protoplasmic  gel  membranes  play  a  vital  part  in  the 
economy  of  the  living  ceU. 

Diffusion. — A  very  important  difference  between 
crystalloids  and  colloids,  a  difference  used  by  Thomas 
Graham,  who  first  investigated  the  subject  in  the 
middle  of  last  century,  to  distinguish  between  them 
is  their  relative  diffusibility. 


DIFFUSION  55 

If  a  strong  (concentrated)  solution  of  a  crystalloid 
is  brought  into  contact  with  a  weak  (dilute)  solution 
of  the  same  crystalloid  in  the  same  solvent,  diffusion 
proceeds  until  the  solute  is  equally  distributed  through 
the  whole  of  the  liquid.  This  is  an  expression  of 
the  general  physical  law  that  all  systems  tend  towards 
equilibrium.  The  rate  of  diffusion  varies  directly  with 
the  difference  in  concentration  of  the  two  solutions, 
and  with  the  temperature,  inversely  with  the  size  of 
the  molecule  of  the  solute.  If  now  the  two  solutions 
of  different  concentrations  are  separated  by  a  colloidal 
membrane  of  the  nature  of  a  gel,  such  as  vegetable  parch- 
ment, crystalloid  solutes  will  diffuse  through  the 
membrane  as  they  would  diffuse  into  a  liquid  with 
which  they  were  directly  in  contact,  though  less 
rapidly.  The  rate  of  this  diffusion  through  a  membrane 
depends  partly  on  the  nature  of  the  membrane,  and 
partly  on  the  size  of  the  molecule  of  the  solute,  partly 
again  on  the  chemical  relation  between  the  solute  and 
the  membrane. 

The  disperse  phase  of  a  colloid  sol,  on  the  other  hand, 
does  not  in  general  pass  through  a  gel  membrane  at 
all,  or  only  does  so  with  extreme  slowness,  and  this 
again  depends  on  the  size  of  the  particles,  on  the  nature 
of  the  membrane,  and  on  the  chemical  relations 
between  them.  It  was  this  difference  which  Graham 
used  to  distinguish  colloids  from  crystalloids,  and  he 
showed  that  a  colloid  and  a  crystalloid  in  mixed  solution 
could  be  separated  by  placing  the  solution  in  a  parch- 
ment bag  and  plunging  the  bag  into  water,  when  the 
whole  of  the  crystalloid  would  eventually  escape  through 
the  membrane,  leaving  the  colloid  pure  behind.  This 
process  is  called  dialysis. 

We  may  say,  broadly,  that  crystalloids  in  solution 


56      PHYSICAL   CHARACTERS   OF   ORGANIC   SUBSTANCES 

will  dialyse,  and  that  colloids  will  not.  But  different 
crystalloid  solutes  pass  through  a  given  membrane  at 
very  different  rates.  The  membrane  may  be  likened 
to  a  sieve  with  meshes  of  definite  size,  though  varying 
within  limits.  These  meshes  will  let  through  molecules 
up  to  that  size,  but  not  larger  ones.  The  disperse 
particles  of  a  colloid  sol  are  in  general  too  large  to  pass 
through  the  membrane,  but  in  some  cases  they  may 
be  just  small  enough  to  pass  in  small  numbers.  The 
matter  is  further  complicated,  as  we  have  seen,  by  the 
reaction  in  some  cases  between  the  molecules  of  the 
solute  or  disperse  phase  and  those  of  the  membrane. 

A  membrane  which  will  allow  a  solute  to  pass  through 
it  is  said  to  be  permeable  to  the  solute.  But  a  mem- 
brane which  is  permeable  to  the  solvent  and  not  to 
the  solute  is  said  to  be  semi-permeable.  Different 
colloidal  membranes  show  very  different  degrees  of 
permeability  to  different  crystalloid  solutes,  some 
letting  through  a  large  variety  of  solutes,  others  being 
impermeable  to  many,  while  others  are  impermeable 
to  practically  all  solutes,  though  they  allow  water  to 
pass.  This  fact  has  a  great  importance  in  the  living 
cell  because  the  gel  membrane  or  film  on  the  surface  of  the 
layer  of  protoplasm  which  lines  the  cell  wall  is  a  semi- 
permeable  membrane,  allowing  certain  solutes  to  pass 
quickly,  others  slowly,  and  others,  again,  not  at  all. 

Osmosis. — If  we  place  some  sugar  solution  in  a 
parchment  bag  (i.e.  a  colloidal  membrane)  open  at 
the  top,  and  immerse  the  bag  in  water  up  to  the  level 
of  the  sugar  solution  within  the  bag,  we  find  that  the 
level  of  the  liquid  within  the  bag  gradually  rises  owing 
to  the  passage  of  water  from  the  beaker  through  the 
membrane,  while  the  sugar  does  not  pass  out  into  the 
beaker.  The  membrane  is  said  to  be  "  semi-permeable 


OSMOSIS  57 

to  sugar  solution,"  the  molecules  of  the  solute  being 
unable  to  pass  through  it,  while  the  molecules  of  water 
can.  If  common  salt  solution,  instead  of  sugar,  is 
within  the  bag,  the  molecules  of  salt  can  pass  out 
(though  at  first  the  liquid  rises  in  the  bag  because 
the  water  passes  in  more  quickly  than  the  salt  can 
pass  out),  since  the  membrane  is  permeable  to  them. 
The  solution  within  the  bag  becomes  less  concentrated 
owing  to  the  entrance  of  water  and  the  escape  of  salt, 
while  the  liquid  outside  becomes  a  salt  solution  of 
increasing  strength  till  the  two  are  of  equal  concentration 
and  equilibrium  is  attained.  But  in  the  case  of  the 
sugar  equilibrium  cannot  be  attained  in  this  way 
because  the  sugar  molecules  cannot  escape  through 
the  membrane,  and  so  the  water  continuously  enters, 
rising  in  the  bag  if  it  is  open  at  the  top,  distending  its 
extensible  wall  if  the  bag  is  closed.  This  process  of 
the  passage  of  a  solvent  through  a  semi-permeable 
membrane  which  will  not  allow  the  solute  to  pass  is 
called  osmosis,  and  the  pressure  developed  on  the  wall 
of  the  membrane  is  known  as  osmotic  pressure. 

Various  solutes,  among  which  sugars  are  prominent, 
to  which  the  protoplasmic  membranes  bounding  the 
cells  appear  to  be  practically  impermeable,  are  formed 
within  the  living  cells  of  plants  and  animals.  These 
are  known  as  osmotic  substances,  and  many  of  the  pro- 
cesses of  absorption  and  movement  of  water  within 
the  organism  depend  on  the  passage  of  water  through 
membranes  on  the  other  side  of  which  such  solutes 
exist,  in  accordance  with  the  universal  tendency  to 
establish  equilibrium. 

We  now  begin  to  see  the  importance  of  the  fact  that 
the  structure  of  organisms  is  so  largely  built  up  of 


58      PHYSICAL  CHARACTERS   OF  ORGANIC   SUBSTANCES 

colloidal  substances.  On  the  one  hand  we  have  the 
power  of  the  colloid  gel  to  absorb  water,  and  the  power 
of  both  gels  and  sols  to  adsorb  a  great  variety  of 
substances  as  a  result  of  their  immense  internal  surfaces. 
On  the  other  we  have  the  fact  that  the  gel  membrane, 
especially  the  protoplasmic  membrane,  is  semi-perme- 
able, so  that  it  does  not  allow  of  the  escape  of  osmotic 
substances  within  the  cell,  while  it  does  allow  water 
to  enter  freely,  and  other  substances,  such  as  salts, 
at  various  rates. 

The  colloidal  nature  of  complex  organic  substances 
largely  depends  on  the  great  size  of  their  molecules, 
and  this  is  most  conspicuously  seen  in  the  proteins 
which  form  the  basis  of  protoplasm.  The  great  size 
of  these  complex  molecules  is  often  associated  with 
instability  of  particular  arrangements  of  the  atoms 
which  compose  it.  This  makes  possible  the  existence 
of  very  numerous  chemical  compounds  of  distinct 
though  closely  similar  molecular  structure.  A  given 
group  of  atoms  within  the  molecule  is  replaced  by 
another  of  somewhat  different  arrangement.  This  is 
the  cause  of  the  extraordinary  richness  and  variety 
of  the  chemical  and  physical  changes  within  the  cell 
which  are  the  basis  of  the  varied  manifestations  of 
life  and  of  organic  form. 

Though  we  are  still  very  far  from  understanding 
exactly  how  what  we  call  living  protoplasm  actually 
carries  out  all  the  complex  processes  which  take  place 
within  it,  we  can  already  trace  the  direct  dependance 
of  many  of  them  on  the  chemical  and  physical 
structure  and  relations  of  the  substances  of  which 
protoplasm  is  composed. 


CRYSTALLOIDS   AND   COLLOIDS  59 

PRACTICAL   WORK. 

A.  BEHAVIOUR  OF  CRYSTALLOIDS  AND  COLLOIDS  WITH  WATER. 

(1)  Place  some  crystals  of  potassium  bichromate  at  the  bottom 
of  a  test  tube  and  just  cover  with  hot  water.     The  crystals 
quickly  dissolve  and  disappear  and  there  is  no  expansion  on 
solution.     Examine  a  drop  of  the  solution  under  the  microscope — 
it  is  clear,  no  particles  are  to  be  seen.     This  is  a  true  (crystalloid) 
solution.     Leave  the  tube  of  solution  and  examine  later :  as  the 
solution  cools  and  the  solvent  evaporates,  crystals  of  the  salt 
reappear. 

(2)  Warm  some  powdered  glue  in  a  test  tube  with  a  little  water : 
the  glue  disappears  gradually,  taking  up  water  and  forming  a 
colloid  sol.     On  cooling  and  standing  a  solid  jelly  (gel)  is  formed. 

(3)  Place  a  rectangular  strip  of  dry  gelatine  on  a  sheet  of  glass 
over  squared  paper.     Mark  the  edges  of  the  gelatine  with  Indian 
Ink  opposite  the  divisions  of  the  paper.    Allow  the  ink  to  dry, 
then  dip  the  gelatine  in  water  and  float  it  on  the  glass  in  a  little 
water.     Note  the  gradual  expansion  of  the  gelatine  as  it  absorbs 
water.     After  a  time  measure  the  increase  in  area  of  the  gelatine 
strip    by   again   placing   the   glass   over   squared    paper.     Now 
transfer  the  gelatine  to  a  piece  of  fine  muslin  and  hang  it  up  to 
dry.     It  contracts,  and  gradually  decreases  in  area  owing  to  loss 
of  water.     The  gelatine  is  a  colloid  gel. 

(4)  Put  one  drop  of  Indian  Ink  in  a  test  tube  and  dilute  with 
water  till  you  can  see  through  the  mixture.     Filter,  and  examine 
a  drop  of  the  filtrate  under  the  high  power  of  the  microscope. 
It  is  full  of  fine  particles.     This  is  a  coarse  suspensoid  sol.     The 
particles  show  Brownian  movement.     The  particles  do  not  settle 
if  the  tube  is  left  standing. 

(5)  Place  a  very  little  solid  Congo  Red  in  water.     It  apparently 
dissolves,  the  solution  looks  clear  and  the  microscope  reveals 
no  particles.     This  is  a  fine  suspensoid  sol  of  which  the  disperse 
particles  are  ultramicroscopic. 

(6)  Examine  the   demonstration    in   which  the  three   liquids 
(a)  potassium  bichromate  solution,  (b)  Congo  Red  sol,  (c)  Indian 
Ink   sol   are   illuminated   laterally  by  a  strong  beam  (Tyndall's 
beam),     (a)  is  clear,  (b)  and  (c)  cloudy;   (b)  looks  more  cloudy 
when  thus  lighted  than  when  looked  at  against  the  light. 

B.  DIFFERENCE  IN  THE  RATES  OF  DIFFUSION  OF  CRYSTALLOIDS 

AND  COLLOIDS. 

(7)  The  two  test  tubes  provided  contain  at  the  bottom  orange 
and  red  jellies  respectively.     The  orange  jelly  is  impregnated 


60   PHYSICAL  CHARACTERS  OF  ORGANIC  SUBSTANCES 

with  potassium  bichromate,  the  red  with  Congo  Red.  Half 
fill  the  tubes  with  water  and  view  against  a  sheet  of  white  paper. 
The  potassium  bichromate  diffuses  into  the  water  quickly,  the 
Congo  Red  very  slowly. 

(8)  Another  test  tube  contains  jelly,  the  bottom  layer  of  which 
contains  sodium  carbonate,  while  the  top  layer  does  not.  Pour  a 
little  hydrochloric  acid  on  the  top  of  the  jelly,  it  diffuses  down 
through  the  substance.  When  the  layer  containing  sodium 
carbonate  is  reached  by  the  hydrochloric  acid  a  reaction  takes 
place,  bubbles  of  carbon  dioxide  appearing  in  the  jelly.  [Note. — 
The  jelly  must  be  made  up  immediately  before  the  class,  or  the 
sodium  carbonate  will  diffuse  up  into  the  top  layer  of  jelly,  and 
the  reaction  will  take  place  immediately  the  hydrochloric  acid 
enters  the  jelly.] 

C.  COAGULATION  OF  PROTEIN  COLLOID  BY  HEAT. 
(o)  Place  small  portions  of  fresh  white  of  egg  in  two  test  tubes. 
Add  a  little  water  to  one :  the  water  is  absorbed  by  the  colloid. 
Slowly  heat  the  other:  the  white  of  egg  sets  to  a  solid.  Cool 
and  add  a  little  water.  The  coagulated  protein  no  longer  absorbs 
water,  having  lost  its  colloid  structure. 

D.  FORMATION  OF  COLLOID  MEMBRANE. 

(10)  Carefully  drop  a  crystal  of  copper  acetate  into  a  test 
tube  containing  a  2  per  cent,  solution  of  potassium  ferrocyanide. 
A  double  decomposition  occurs  as  the  two  salts  meet  and  the 
copper  ferrocyanide  is  precipitated  to  form  a  colloidal  membrane 
around  the  crystal.  This  membrane  enlarges  by  breaking  and 
patching  so  that  it  seems  to  grow. 

E.  SWELLING  OF  VEGETABLE  COLLOIDS  IN  WATER. 
(u)  Cover  with  water  on  a  sheet  of  glass  the  rectangular  piece 
of  seaweed  [Chondrus  is  very  suitable]  provided,  being  careful 
to  wet  both  sides.     Note  the  expansion  as  with  the  sheet  of 
gelatin  in  (3). 

(12)  Add  water  to  a  dried  section  of  seaweed  under  the  micro- 
scope and  note  the  expansion  that  takes  place. 

(13)  Compare  the  size  and  shape  of  dry  peas  with  those  of 
peas  that  have  been  soaked  in  water  for  24  hours.     Place  six 
soaked  and  six  unsoaked  peas  in  rows  touching  one  another  : 
measure  the  aggregate  diameter  of  the  six  in  each  case  and 
determine  the  average  absolute  and  percentage  increase  in  the 
soaked  peas. 


CHAPTER  IV 

PROTOPLASM  AND  THE  AMCEBA. 
PROTOCOCCUS 

Constitution  and  Structure  of  Protoplasm. — The  vis- 
cous but  semi-liquid  mobile  protoplasm  that  forms 
the  bulk  of  many  animal  and  plant  cells  is  to  be  regarded 
as  a  mixed  colloid  sol.  The  continuous  phase  is  water 
in  which  various  organic  and  inorganic  substances 
are  dissolved.  These  solutes  may  vary  to  a  consider- 
able extent  according  to  circumstances.  The  disperse 
phase  we  believe  to  consist  essentially  of  ultramicro- 
scopic  particles  of  protein,  which  may  possibly  include 
separate  protein  molecules,  as  well  as  aggregations  of 
molecules.  Fats  or  fat-like  bodies  may  also  exist  as 
part  of  the  disperse  phase,  and  these  may  be  closely 
associated  with  the  protein  droplets.  In  this  sol 
numerous  chemical  and  physical  processes  are  con- 
stantly taking  place,  and  it  is  the  sum  of  these  that 
give  rise  to  the  chemical  phenomena  of  life. 

Besides  the  ultramicroscopic  disperse  particles,  which, 
together  with  the  continuous  watery  phase,  form,  so 
far  as  we  can  tell,  the  ultimate  structural  basis  of  living 
protoplasm  as  a  form  of  matter,  the  protoplasm  ordinarily 
contains  much  larger  microscopically  visible  granules 
and  droplets  of  very  various  size  and  very  various  com- 
position. The  presence  of  these  last,  like  that  of  the 
solutes,  depends  upon  the  state  of  the  cell  in  regard  to 
nutrition,  etc.  The  sum  total  of  the  chemical  processes 

61 


62    PROTOPLASM  AND  THE  AMCEBA.   PROTOCOCCUS 

going  on  in  the  living  protoplasm  is  called  metabolism,1 
and  these  larger  granules  and  droplets  are  metabolites, 
i.e.  products  of  metabolism.  They  must  be  carefully 
distinguished  from  the  ultramicroscopic  disperse  particles 
which  we  cannot  see,  but  whose  existence  we  confidently 
infer  from  different  lines  of  evidence,  and  which  form 
the  permanent  structure  of  protoplasm. 

In  the  clear  protoplasm  of  the  pseudopodium  of  the 
amoeba  (see  Fig.  i),  in  which  there  are  no  coarse  granules, 
very  numerous  fine  particles  can  be  seen  in  Brownian 
movement.  This  movement  is  suddenly  brought  to 
an  end  by  electrical  stimulation.  When  the  stimula- 
tion ceases  the  Brownian  movement  recommences.  The 
mobile  sol  has  been  momentarily  converted  into  a 
reversible  gel.  When  protoplasm  is  killed  by  certain 
"  poisons  "  (what  are  called  in  biology  "  fixing  reagents," 
such  as  acetic  acid,  chromic  acid  or  dilute  iodine  solu- 
tion, etc.)  the  sol  is  converted  into  an  irreversible  gel. 
If  protoplasm  is  heated  to  60°  C.,  however,  chemical 
changes  occur  in  the  proteins,  and  the  whole  colloid 
structure  is  destroyed.  The  proteins  are  then  said  to 
be  "  coagulated." 

The  protein  particles  forming  the  disperse  phase  of 
the  protoplasmic  sol  very  readily  run  together  to  form 
a  membrane  (gel)  in  any  plane  on  the  two  sides  of  which 
they  are  subjected  to  different  physico-chemical  action. 
This  occurs  for  instance  on  the  outer  surface  of  a 
detached  unit  of  protoplasm  such  as  a  cell,  and  this  is 
accordingly  covered  with  a  semi-permeable  protoplasmic 
membrane,  the  ectoplasm,  which  is  relatively  fixed  and 
sharply  distinguished  from  the  rest  of  the  cell  proto- 
plasm (endoplasm) .  Similar  gel  membranes  are  formed 
round  the  vacuoles  (segregated  drops  of  liquid  enclosed 
1  Greek  neta^o\r\,  a  changing  over. 


CHANGES  IN  AGGREGATION  OF  PROTOPLASM    63 

in  the  endoplasm),  and  also  round  the  cell  nucleus. 
It  will  be  noted  that  all  these  phenomena  of  the  living 
cell  are  strictly  in  accordance  with  what  we  learned  of 
the  behaviour  of  non-living  colloids  in  the  last  chapter. 
The  special  characters  of  protoplasm  are  largely 
dependent  on  the  fact  that  it  has  the  power  of  readily 
passing  from  the  sol  to  the  gel  condition  and  back  again. 
Normally  it  may  be  said  to  exist  somewhere  near  the 
margin  of  the  sol  and  the  gel  conditions.  About  o°  C.  it 
gels  and  becomes  inactive,  though  as  we  shall  see  in 
later  chapters  it  is  not  "  killed,"  i.e.  its  essential  struc- 
ture is  not  destroyed,  at  far  lower  temperatures.  As 
the  temperature  rises  it  becomes  more  labile  and  more 
active,  till  above  40°  C.  changes  begin  to  occur  which 
eventually  destroy  its  structure  altogether.  Many  other 
causes  besides  changes  of  temperature  and  the  loss  or 
absorption  of  water  send  the  protoplasm  from  the  sol  to 
the  gel  condition  and  back  again,  and  it  is  on  the  changes 
in  aggregation  of  the  protein  molecules  and  groups  of 
molecules,  the  association  and  dissociation  of  these  with 
one  another,  and  with  molecules  of  fats  and  salts,  that 
what  we  call  the  vital  activities  of  the  protoplasm 
depend.  The  more  aggregation  occurs,  the  more  gel- 
like  and  relatively  stable  the  structure  of  the  protoplasm 
becomes,  the  less  free  the  motion  of  particles  and 
molecules  from  one  part  of  the  living  cell  to  another. 
Consequently  different  chemical  and  physical  processes 
can  go  on  in  different  parts  of  the  cell  (as  is  often 
the  case)  without  interfering  with  one  another.  Dis- 
aggregation  leads  to  the  opposite  effect.  Many  of  the 
more  specialised  animal  cells  have  their  protoplasm 
more  or  less  permanently  in  the  gel  condition,  and  the 
specialised  character  of  their  activities  depends  largely 
on  this  fact.  But  on  the  whole  the  life  activities 


64   PROTOPLASM  AND  THE  AMCEBA.   PROTOCOCCUS 

depend  essentially  on  the  maintenance  of  the  proto- 
plasm within  a  comparatively  narrow  range  of  mole- 
cular aggregation,  and  on  its  power  of  passing  backwards 
and  forwards  within  this  range. 

Since  the  watery  continuous  phase  of  protoplasm  is 
an  essential  part  of  its  structure,  the  equilibrium  of 
the  protoplasmic  water  must  be  maintained  or  the 
structure  of  protoplasm  would  be  destroyed.  In 
other  words,  the  forces  tending  to  drive  water  out  of 
the  cell  and  those  tending  to  draw  water  into  the  cell 
must  be  balanced.  This  condition  of  balance  is  main- 
tained by  primitive  organisms  living  in  water  and 
consisting  of  naked  protoplasm,  when  they  are  sur- 
rounded by  water.  If  such  an  organism  is  transferred 
to  air,  evaporation  from  the  surface  into  the  air  at  once 
destroys  the  water  balance,  the  body  loses  water  and 
the  organism  dies.  This  is  why  terrestrial  organisms 
which  live  surrounded  by  air  must  have  some  means 
of  checking  the  loss  of  water  by  evaporation  and  of 
renewing  it  as  it  is  lost.  We  shall  see  in  later  chapters 
that  the  history  of  evolution  of  land  plants  is  very 
largely  a  history  of  the  appearance  and  development 
of  structures  which  have  these  results. 

The  Amoeba. — The  most  suitable  organism  for  pre- 
liminary study  is  the  simple  minute  animal  called  the 
amoeba.  There  are  various  species  which  live  in  fresh- 
water pools  and  slow  rivers,  where  they  creep  about 
upon  the  surface  of  the  mud  or  upon  the  water  plants. 

Each  individual  consists  of  a  minute  irregular  mass 
of  naked  protoplasm  varying  in  different  species  from 
about  100  /u,  to  about  250  /*  in  diameter.1  The  surface 
layer  of  the  body  (ectoplasm)  is  clear  under  the  micro- 

1  I.e.  from  about  -fa  to  %  of  a  millimeter,  or  about  ^io  t°  TSS  °f 
an  inch. 


AMCEBA  65 

scope,  the  rest  of  the  protoplasm  is  filled  with  granules 
visible  under  the  microscope  and  very  variable  in  size 
and  nature.  In  the  ectoplasm,  but  bulging  into  the 
endoplasm,  is  situated  a  spherical  space  containing 
liquid,  and  called  the  contractile  vacuole,  because  it  can 


B 


FIG.  i. — Amoeba  proteus.  Two  successive  views  of  the  same  individual. 
The  arrows  show  the  direction  of  movement  of  the  pseudopodia, 
which  in  A  are  adhering  to  the  glass  slide.  PS.,  pseudopodium ; 
ect.,  ectoplasm  (clear) ;  end.,  endoplasm  (granular)  ;  n,  nucleus ; 
c.v.,  contractile  vacuole;  d,  diatoms  (unicellular  plants),  which 
have  been  taken  into  the  endoplasm  as  food.  The  larger  granules 
are  the  undigested  remains  of  food.  After  Cash,  x  about  320 
(the  organism  is  just  visible  to  the  naked  eye). 

be  observed  to  contract  till  it  disappears  completely 
or  almost  completely,  and  then  quickly  appears  again 
in  the  same  spot.  The  contractile  vacuole  is  generally 
regarded  as  excretory  in  function,  i.e.  as  forcing  out  of 
the  body,  whenit  contracts,  waste  products  of  metabolism 
including  urea  (a  waste  product  containing  nitrogen), 


66   PROTOPLASM  AND  THE  AMCEBA.   PROTOCOCCUS 

and  perhaps  also  carbon  dioxide.  Thus  its  function  is 
equivalent  to  that  of  the  kidneys  and  bladder  of  the 
higher  animals,  and  partly  perhaps  to  that  of  the 
lungs. 

In  the  endoplasm  is  a  rounded  body  called  the 
nucleus,  which,  as  we  shall  see  presently,  is  an  all 
important  part  of  the  protoplasm.  The  rest  of  the 
protoplasm  (ectoplasm  and  endoplasm  together)  is 
called  cytoplasm.  Nucleus  and  cytoplasm  together  form 
the  structural  unit  of  living  organisation  in  the  vast 
majority  of  plants  and  animals.  Such  a  unit  is  called 
a  cell,  and  the  amoeba  is  commonly  called  a  unicellular 
animal  because  it  consists  of  one  such  unit,  in  contrast 
with  multicellular  animals  and  plants  which  consist  of 
many,  in  the  largest  forms  of  many  millions. 

The  nucleus  is  covered  with  a  nuclear  membrane  (gel 
membrane)  and  contains  a  substance  chromatin,  which 
can  usually  be  seen  under  the  microscope  in  the  form 
of  granules.  The  chromatin  J  is  distinguished  by  its 
power  of  taking  up  and  becoming  coloured  by  dyes 
(stains),  particularly  certain  stains  ("  nuclear  stains  "), 
with  great  readiness.  This  probably  takes  place  by 
the  adsorption  of  the  particles  of  the  dye  by  the  dis- 
perse phase  of  the  gel  of  which  the  chromatin  con- 
sists. Chemically  the  chromatin  is  distinguished  by  the 
presence  in  it  of  complex  proteins  containing  phosphorus. 

The  function  of  the  nucleus  in  the  life  of  the  cell 
can  be  inferred  from  its  behaviour  and  from  the  be- 
haviour of  the  cytoplasm  in  relation  to  it  under  various 
conditions.  Certain  large  Protozoa  (unicellular  animals) 
can  be  cut  into  pieces  under  the  microscope  without 
killing  them.  When  this  is  done  and  one  part  of  the 
divided  cell  contains  the  nucleus  while  the  others  do 
1  Greek  &Jia,  colour. 


FUNCTION   OF   THE   NUCLEUS  67 

not,  the  part  containing  the  nucleus  regenerates,  i.e. 
it  grows  again  into  a  complete  cell.  The  parts  not 
containing  the  nucleus  remain  for  a  time  sensitive  and 
motile,  but  they  cannot  assimilate  food,  and  hence 
they  ultimately  die.  In  living  plant  cells,  when  the 
protoplasm  is  in  a  state  of  great  activity  and  metabolism 
is  proceeding  rapidly,  the  nucleus  is  large  and  con- 
spicuous (Fig.  43,  A,  B,  C),  but  when  the  cell  is  com- 
paratively inactive  the  nucleus  is  usually  small  and 
inconspicuous.  Again,  when  local  activity  is  going  on 
in  a  cell  the  nucleus  commonly  moves  to  the  spot  in 
which  work  is  being  carried  out.  For  instance,  if  a 
plant  cell  is  thickening  its  cell  wall  in  one  spot  the 
nucleus  moves  to  the  spot  where  the  thickening  is 
going  on  and  remains  there  while  this  activity  is  in 
progress  (Fig.  43,  E).  It  appears  to  be  "  superintending 
operations." 

From  facts  like  these  we  infer  that  the  nucleus  is 
essential  in  directing  the  metabolic  and  formative  cell 
processes.  Exactly  what  it  does  we  do  not  know,  but 
very  likely  it  sends  out  into  the  cytoplasm  chemical 
substances,  perhaps  enzymes,  which  are  essential  in 
carrying  out  the  work  of  the  protoplasm. 

The  chromatin  of  the  nucleus  is  certainly  different 
in  every  species  of  organism,  and  even  in  those  smaller 
groups  of  very  uniform  individuals  within  the  species 
which  are  called  "  pure  strains."  The  differences 
probably  depend  on  the  existence  in  the  chromatin  of 
slightly  different  proteins  (cf.  p.  43).  The  cytoplasm 
of  different  species  and  strains  also  probably  differs  in 
the  same  sort  of  way,  but  we  have  very  good  evidence 
that  the  differences  of  the  chromatin  are  the  most 
important.  The  peculiar  formative  powers  of  each 
species,  the  powers  that  enable  it  to  reproduce  the 


68   PROTOPLASM  AND  THE  AM(EBA.   PROTOCOCCUS 

exact  specific  form  of  body  and  function,  are  certainly 
carried  mainly,  if  not  entirely,  by  the  chromatin  in 
reproduction,  in  which  the  nuclei  of  the  reproductive 
cells  play  the  leading  part.  Often  indeed  the  nucleus 
of  one  of  the  parent  cells  of  the  new  organism  is  prac- 
tically the  only  thing  contributed  by  that  parent 
to  the  new  organism,  which  may  nevertheless  exhibit 
some  of  the  characteristics  of  that  parent  in  a  perfectly 
pure  form. 

Life  Processes  of  Amoeba. — The  most  obvious  thing 
the  amoeba  does  is  to  move.  When  the  animal  is  active 
its  shape  continually  changes,  the  clear  ectoplasm 
thrusts  out  from  the  body  and  is  followed  by  the  stream- 
ing granules  of  the  endoplasm.  The  protoplasm  of 
the  projection  so  formed  (pseudopodium)  glides  along 
the  surface  to  which  the  amoeba  adheres,  and  may  be 
followed  by  the  streaming  forward  of  the  rest  of  the 
body,  which  thus  gradually  glides  away  from  the 
position  it  occupied  at  first.  Thus  the  animal  con- 
stantly changes  its  shape  I  and  at  the  same  time  its 
position.  Locomotion  (movement  from  place  to  place) 
follows  as  an  immediate  result  of  the  streaming  move- 
ment of  the  protoplasm  in  a  constant  direction.  The 
protrusion  of  pseudopodia  is  not,  however,  necessanly 
followed  by  locomotion  :  several  pseudopodia  may  be 
put  out  at  different  spots  on  the  surface  of  the  body 
and  again  withdrawn,  so  that  no  continuous  streaming 
in  one  direction  occurs.  Different  species  of  amoeba 
vary  very  considerably  in  the  shape  and  dimensions 
of  their  pseudopodia. 

The  streaming  movement  of  the  pseudopodia  is  also 
one    method    by    which    the    amoeba  feeds.     When   a 
pseudopodium  touches  another  living  organism  much 
»  The  name  amoeba  is  derived  from  Greek  duoifirj,  change. 


INGESTION   AND   DEFECATION  69 

smaller  than  the  amoeba,  for  instance  a  unicellular 
green  plant  (alga)  or  a  colony  of  bacteria,  the  proto- 
plasm flows  round  it  or  draws  it  into  the  body,  so  as 
completely  to  enclose  the  prey.  With  the  prey  a  drop 
of  water  may  be  ingested,  forming  a  food  vacuole.  The 
food  is  then  acted  upon  by  enzymes  formed  by  the 
amoeba's  protoplasm,  and  the  digestible  parts  digested, 
i.e.  broken  down  into  soluble  organic  substances, 
probably  proteins,  carbohydrates  and  fats,  which  can 
be  assimilated  by  the  amoeba,  i.e.  built  up  again  into 
new  "  amoeba-protoplasm,"  while  the  indigestible  parts 
are  ejected  from  the  body  as  faces.  This  process  of 
defalcation  is  sometimes  loosely  called  "  excretion," 
but  it  must  be  carefully  distinguished  from  the  excretion 
of  urea  and  carbon  dioxide  which  are  formed  by  the 
chemical  breaking  down  of  the  proteins  and  carbo- 
hydrates of  the  body  The  ingestion  of  the  prey  when 
the  pseudopodium  has  once  come  into  contact  with  it, 
and  also  the  ejection  of  faeces,  can  be  explained  by  the 
purely  physical  force  of  surface  tension,  for  it  has  been 
shown  that  a  drop  of  chloroform  (representing  the 
amoeba)  placed  in  contact  with  a  minute  fibre  of  glass 
covered  with  a  varnish  of  shellac  will  ingest  the  fibre, 
remove  the  coating,  and  then  eject  the  fibre  (see  Fig.  2). 
A  drop  of  chloroform  will  also  coil  up  a  flexible  fibre 
of  shellac  within  itself  just  as  an  amoeba  will  coil  up 
a  flexible  algal  thread  (Fig.  3). 

By  the  formation  of  new  protoplasm  the  amoeba 
naturally  adds  to  its  bulk  and  thus  grows  in  size,  unless 
of  course  the  breaking  down  processes  (katabolism) 
which  result  in  excretion  of  the  katabolites  balance  or 
exceed  the  building  up  or  assimilative  processes. 

Amoeba  is  also  sensitive  to  external  stimuli,  i.e.  its 
motion  is  directed  in  relation  to  such  stimuli.  For 


70   PROTOPLASM  AND  THE  AMCEBA.   PROTOCOCCUS 

instance,  its  protoplasm  streams  away  from  a  source 
of  heat  when  the  temperature  rises  above  35°  C.  At 
o°  C.  and  at  35°  C.,  or  in  the  absence  of  sufficient  dis- 


FIG.  2. — Spreading  of  a  drop  of  chloroform  over  a  shellac-covered 
glass  fibre.  Contact  with  the  shellac  lowers  the  surface  tension 
of  the  drop  of  chloroform,  which  spreads  over  and  draws  the 
fibre  into  itself,  removing  the  shellac.  When  the  glass  fibre  is 
bare  the  chloroform  recovers  its  surface  tension,  reassumes  the 
spherical  form  and  thus  expels  the  fibre,  which  it  cannot  "  wet." 
The  arrows  in  a,  c  and  d  show  direction  of  movement  of  the 
fibre;  in  b  the  movement  of  the  chloroform;  S,  shellac  in  the 
chloroform.  After  Rhumbler. 


SENSITIVENESS   OF   AMOIBA  71 

solved  oxygen  in  the  water,  the  pseudopodia  are  with- 
drawn and  the  animal  becomes  spherical  and  motion- 
less. Amoeba  is  also  sensitive  to  the  presence  of  food, 
for  it  does  not  normally  ingest  objects  which  it  cannot 
digest,  and  this  probably  means  that  chemical  substances 


B 


FIG.  3. — A,  ingestion  and  coiling  up  of  a  thread  of  Oscillaria  (an 
alga)  by  Amceba  verrucosa.  B,  ingestion  and  coiling  up  of  a 
thread  of  shellac  by  a  drop  of  chloroform.  After  Rhumbler. 


diffusing  out  from  the  organisms  which  form  its  prey 
come  into  contact  with  the  ectoplasm  of  the  amoeba, 
and  this  stimulus  leads  to  the  envelopment  of  the 
prey. 

Reproduction  of  Amoeba. — When  the  growth  of  the 
individual  amoeba  has  reached  a  certain  limit  the  animal 


72   PROTOPLASM  AND  THE  AMOEBA.   PROTOCOCCUS 

divides  into  two.  The  nucleus  first  divides  into  two 
equal  parts,  and  then  a  furrow  appears  in  the  cytoplasm 
and  rapidly  deepens  till  the  two  halves  become  entirely 
separate.  This  simple  method  of  multiplication  is 
called  binary  fission.  Each  of  the  halves  is  a  complete 
amoeba  in  every  respect,  and  each  proceeds  to  lead  its 
own  independent  life  and  ultimately  grows  to  the  size 
of  the  parent  amoeba,  when  it  divides  again  in  the 
same  way.  [The  amoeba  has  other  modes  of  reproduction 
which  will  not  be  dealt  with  here.] 
Protococcus. — On  the  bark  of  trees,  on  old  palings, 


FIG.  4. — Cells  of  Protococcus  viridis.  x  about  800.  Each  is  bounded 
by  a  colourless  cell  wall  (c.w.).  The  shaded  portion  is  the  green 
chloroplast  which  frequently  occupies  most  of  the  space  within 
the  wall  (ft  and  c) ;  pyr.,  pyrenoid.  In  a  and  d  the  cells  have  recently 
divided  and  are  remaining  together. 

etc.,  especially  on  the  north  side,  which  does  not  en- 
counter the  direct  rays  of  the  sun  and  thus  tends  to 
remain  damp,  one  often  finds  the  surface  covered  with 
a  thin  green  crust,  which  crumbles  in  dry  weather 
under  the  point  of  a  knife.  If  a  little  of  this  green 
crust  is  scraped  off  into  a  drop  of  water  on  a  slide  and 
examined  first  with  the  low  and  then  with  the  high 
power,  it  is  seen  to  consist  of  numerous  very  small 
green  cells,  which  often  hang  together  in  groups  of 
two,  four,  or  more.  This  is  a  minute  unicellular  alga 


STRUCTURE   AND   DIVISION   OF   PROTOCOCCUS          73 

called  Protococcus  vulgaris,  which  inhabits  the  surface 
of  the  damp  tree  bark,  and  feeds  by  absorbing  rain 
water  that  falls  on  the  tree  and  trickles  down  over 
the  surface.  This  rain  water  dissolves  carbon  dioxide 
from  the  air  and  small  quantities  of  mineral  salts,  and 
thus  it  contains  all  the  elements  necessary  for  the 
food  of  the  green  alga. 

Under  the  high  power  of  the  microscope  (Fig.  4), 
each  cell  of  Protococcus  is  seen  to  be  bounded  by  a 
colourless  cell  wall  of  firm  consistency.  Inside  this  is 
the  essential  living  part  of  the  organism — the  protoplasm 
of  the  cell.  With  the  low  power  this  appears  uniformly 
green,  but  under  the  high  power  it  can  be  observed 
that  the  green  protoplasm  does  not  fill  the  whole  of 
the  space  within  the  cell  wall,  but  takes  the  form  of 
a  curved  plate  (chloroplast)  in  which  can  often  be 
observed  a  central  shining  body  (pyrenoid),1  the  rest 
of  the  cell  being  occupied  by  colourless  cytoplasm. 
By  appropriate  staining  the  cell  nucleus  can  also  be 
distinguished,  but  it  is  generally  hidden  by  the  massive 
chloroplast. 

Protococcus  reproduces  itself  by  cell  division,  the 
nucleus  dividing  first  and  then  the  chloroplast ;  a 
delicate  wall  is  afterwards  formed  across  the  cell, 
separating  it  into  two  halves  (Fig.  4,  a,  left-hand  cell). 
The  dividing  cell  wall  thickens  and  the  cells  may 
separate,  but  very  frequently  they  remain  together 
in  little  groups.  All  stages  of  cell  division  and  separa- 
tion can  frequently  be  seen. 

If  we  compare  Amoeba  and  Protococcus  as  a  repre- 
sentative simple  animal  and  a  representative  simple 
green  plant,  we  note  first  that  they  resemble  one  another 

1  A  protein  crystal-like  body  found  in  the  chloroplasts  of  many 
green  algae  and  acting  as  a  centre  of  starch  formation. 


74   PROTOPLASM  AND  THE  AMCEBA.   PROTOCOCCUS 

in  both  being  isolated,  independent,  free-living  organ- 
isms, each  consisting  essentially  of  cytoplasm  with 
nucleus.  But  they  show  the  following  differences  : — 

(1)  Amceba  has  no  cell  wall,  while  Protococcus  has. 

(2)  Amaba  moves  from   place  to  place,  while  Pro- 
tococcus is  stationary. 

(3)  Amceba  feeds  on  living  prey,   while  Protococcus 
does  not,  having  no  means  of  ingesting  solid  bodies. 

On  the  other  hand 

(4)  Protococcus  has  part  of  its  cell  protoplasm  coloured 
green  (chloroplast),  which  enables  it  to  form  organic 
substance,  and  eventually  protoplasm,  from  liquid  and 
gaseous    inorganic    substances    which    it    can    absorb 
through    its    cell    wall.     Amceba    has    no    such    power 
because  it  has  no  chloroplast. 

PRACTICAL  WORK. 

A.  AMCEBA  AS  A  TYPE  OF  SIMPLE  ANIMAL. 

(i)  Place  a  drop  of  water  containing  the  scrapings  of  the 
surface  mud  of  a  pond  containing  amcebae  (together  with  other 
organisms  and  organic  debris)  in  the  centre  of  a  slide  and  cover 
with  a  coverslip.  Examine  first  with  the  low  power,  and  pick 
out  the  amcebae,  which  appear  as  irregular  blobs  of  greyish  or 
brownish  appearance.  They  will  gradually  creep  out  from  the 
debris  along  the  surface  of  the  slide  or  coverslip. 

Examine  a  specimen  with  the  high  power  and  notice  that  it 
is  constantly  changing  its  shape,  sometimes  slowly  and  sometimes 
rapidly.  Distinguish  the  clear  ectoplasm  from  the  granular 
endoplasm,  and  note  that  the  granules  of  the  latter  are  very 
various  in  size  and  shape.  The  endoplasm  may  include  quite 
large  remains  of  ingested  prey,  e.g.  the  cell  walls  and  disintegrated 
contents  of  unicellular  plants.  Note  also  the  transient  projections 
of  the  body  (pseudopodia). 

Identify  the  contractile  vacuole,  and  watch  it  contracting  and 
quickly  reappearing  at  full  size.  Identify  also  the  rounded, 
granular,  light-refracting  nucleus.  Measure  the  average  diameter 
of  the  amoeba  and  the  diameter  of  the  nucleus  with  the  eye-piece 
micrometer. 


PRACTICAL   WORK  75 

(2)  Draw  an  active  specimen  in  successive  stages  of  motion 
to  show  the  change  of  shape  depending  on  the  protrusion  and 
withdrawal  of  pseudopodia.     See  if  you  can  observe  an  amoeba 
in  process  of  ingesting  another  organism  as  food. 

(3)  Kill  the  amoeba  by  running  a  drop  of  iodine  or  acetic  methyl 
green  under  the  coverslip  and  note  that  the  nucleus  is  stained  more 
deeply  than  the  cytoplasm. 

(4)  Examine  a  demonstration  specimen  of  amoeba  which  has 
been   fixed,    stained    and    permanently  mounted,    showing    the 
deeply  stained  nucleus. 

B.  PROTOCOCCUS  AS  A  TYPE  OF  SIMPLE  PLANT. 

(1)  Scrape  off  a  little  of  the  green  crust  from  the  bark  of  a 
tree  into  a  drop  of  water  and  cover  with  a  coverslip.     Examine 
with  the  low  power  and  note  the  numerous  green  specks  and 
clumps  in  the  field  of  the  microscope.     Put  on  the  high  power 
and  distinguish  cell  wall,   chloroplast,   and  colourless  cytoplasm. 
The  chloroplast  generally  contains  a  bright  granule,  the  pyrenoid. 
More  than  one  chloroplast  may  exist  in  the  cell.     The  nucleus 
is  usually  difficult  or  impossible  to  distinguish.     It  lies  in  the 
colourless   cytoplasm,    often    in    the   concavity   of    the   curved 
chloroplast. 

(2)  Examine  stages  in  the  division  of  the  cells  (only  found 
when  the  Protococcus  is    kept   damp)    (a)   when   the  new  cell 
wall  is  not  yet  formed  but  the  chloroplasts  of  the  daughter  cells 
are  distinct,  and  (b)  after  the  cross  wall  is  formed  but  the  two 
daughter  cells  are  still  clearly  derived  by  division  of  the  parent 
cell.     Add  a  drop  of  very  dilute  iodine  solution  and  observe  the 
effect. 


CHAPTER    V 

THE  VITAL  FUNCTIONS 

THE  activities  of  a  living  organism  which  together  make 
up  what  we  call  its  "  life  "  may  be  conveniently  con- 
sidered under  separate  heads  which  are  often  called 
"  the  vital  functions."  These  divisions  are  to  some 
extent  artificial,  for  one  process  passes  into  another, 
and  others  are  the  results  of  the  interaction  of  more 
than  one.  We  must  always  remember  that  what  we  call 
the  different  life  processes  in  reality  form  a  continuous 
whole.  They  are  all  expressions  of  the  activity  of 
protoplasm  in  maintaining  its  physico-chemical  equi- 
librium in  relation  to  its  surroundings,  though  these 
expressions  are  strikingly  different  in  different  organisms. 
The  "  vital  functions "  are  those  expressions  which 
we  can  seize  upon  and  give  names  to,  and  which  are 
of  essentially  the  same  nature  in  all  living  organisms, 
whether  animals  or  plants,  simple  or  complex. 

We  must  carefully  distinguish  between  the  activity 
of  the  organism  as  a  whole  and  the  activity  of  the 
protoplasm  itself,  though  the  former  depends  of  course 
directly  upon  the  latter,  since  protoplasm  is  the  only 
living  part  of  an  organism.  This  distinction  is  least 
evident  when  the  whole  body  of  the  organism,  as  in  the 
amoeba,  consists  solely  of  a  naked  mass  of  protoplasm. 
In  multicellular  organisms,  the  relations  of  the  whole 
body  to  its  surroundings  are  necessarily  different  from 
the  relations  of  the  protoplasm  of  each  living  cell  to 


FEEDING  77 

its  surroundings.  This  we  can  illustrate  in  the  case  of 
each  of  the  "  vital  functions." 

(i)  Feeding. — In  the  case  of  amoeba,  what  we  call 
the  food  of  the  animal  is  the  living  prey  which  it  engulfs. 
The  food  of  the  protoplasm,  however,  consists  of  the 
products  of  digestion  of  the  prey  which  can  be  directly 
incorporated  in  the  protoplasm,  or  can  be  broken  down 
to  set  free  energy  without  such  incorporation.  Similarly, 
in  the  case  of  a  higher  animal,  the  food  of  the  animal 
is  the  animal  or  plant,  or  the  part  or  product  of  the 
animal  or  plant,  which  the  animal  eats  ;  the  food  of 
the  protoplasm  of  its  living  cells  consists  of  the  digested 
products  of  the  original  food  which  have  passed  into 
the  blood  and  can  be  absorbed  and  incorporated  into 
their  structure  by  the  living  cells,  or  broken  down 
into  simpler  substances  so  as  to  liberate  energy. 

In  a  green  plant  the  food  of  the  plant  as  a  whole 
consists  of  the  water  and  inorganic  salts  absorbed  by 
the  roots  from  the  soil,  and  of  the  carbon  dioxide 
absorbed  by  the  leaves  from  the  air.  The  food  of  the 
living  protoplasm  of  the  plant  cells,  on  the  other  hand, 
consists  of  the  sugars  and  proteins  built  up  from  these 
simple  substances  taken  in  by  the  plant.  In  the  case 
of  a  germinating  seed  the  food  is  obtained  directly  from 
reserve  stores — starch  or  fats,  which  are  converted  into 
sugar,  and  proteins  which  are  converted  into  a  soluble 
form — that  are  packed  away  in  the  seed.  Here,  it  will 
be  noted,  the  foods  of  the  young  plant  are  of  exactly 
the  same  chemical  nature  as  the  foods  of  an  animal, 
and  they  can  be  used,  and  are  in  fact  largely  used,  as 
food  by  animals,  as  when  we  eat  bread  made  from  the 
reserve  stores  of  the  wheat  seed,  or  beans,  peas  or  nuts, 
which  are  also  seeds  with  large  stores  of  food. 

Thus,  the  food  of  the  organism  as  a  whole  is  very 


78  THE   VITAL   FUNCTIONS 

different  in  different  kinds  of  organism,  though  it  must 
always  contain  the  necessary  chemical  elements,  but 
the  food  of  protoplasm  is  always  of  the  same  nature. 

Four  main  types  of  feeding  are  often  distinguished  : 
holozoic,  in  which  the  organism  ingests  solid  organic 
food  (characteristic  of  animals)  ;  holophytic,  in  which 
the  food  of  the  organism  consists  wholly  of  inorganic 
substances,  including  carbon  dioxide  (characteristic  of 
green  plants)  ;  saprophytic,  in  which  organic  food  is 
obtained  from  the  dead  body  or  from  the  products  of 
some  other  organism  in  a  liquid  form  ;  and  parasitic, 
in  which  organic  food  is  obtained  direct  from  the  living 
body  of  some  other  organism,  but  this  "  host  "  organism 
is  not  immediately  killed.  The  mode  of  nutrition  of 
saprophytic  and  parasitic  organisms  is  not  essentially 
different.  It  is  of  no  essential  importance  whether 
the  liquid  organic  substances,  provided  they  are  suitable 
as  food,  come  from  a  living  or  from  a  dead  body  or 
product ;  and  many  species  of  fungi,  for  instance,  may 
be  both  saprophytic  and  parasitic  (see  Chapter  XI), 
though  in  many  cases  species  are  closely  adapted  to 
one  or  the  other  mode  of  life.  Animal  parasites  may 
have  mouths,  and  suck  or  devour  organic  liquids  or 
tissues  of  the  living  body  of  the  host,  or  they  may, 
like  the  tapeworm,  have  no  mouths,  and  absorb  liquid 
organic  food  through  their  body  wall. 

(2)  Assimilation.1 — This  word  literally  means  "making 
like,"  and  in  its  strict  sense  is  applied  to  the  process 
of  incorporation  of  the  final  foodstuffs  in  the  specific 
protoplasm  of  the  organism.  It  is,  in  fact,  the  actual 
feeding  of  the  protoplasm.  We  know  practically 
nothing  of  how  this  is  done  The  incorporation  of  the 
carbon  derived  from  the  carbon  dioxide  of  the  air  in 

1  Latin  similis,  like. 


ANABOLISM   AND    KATABOLISM — GROWTH  79 

the  molecule  of  sugar,  the  first  stage  in  the  formation 
of  the  protein  molecule  that  goes  to  form  the  structure 
of  protoplasm,  is  often  called  "  carbon  assimilation." 

Anabolism  and  Katabolism. — All  the  constructive 
chemical  processes  which  take  place  in  the  protoplasm 
are  classed  together  as  anabolic  I  processes  or  anabolism, 
as  opposed  to  the  destructive  or  katabolic  3  processes 
(katabolism) .  Thus  the  formation  of  sugar  from  carbon 
dioxide  and  water  in  a  green  plant  is  an  anabolic  process, 
the  breaking  down  of  sugar,  and,  in  the  animal  body, 
the  breaking  down  of  proteins  into  urea  and  uric  acid, 
are  katabolic  processes.  Assimilation  in  the  strict 
sense,  i.e.  the  incorporation  of  foodstuff  in  the  proto- 
plasm, is  the  ultimate  anabolic  process. 

(3)  Growth. — In  its  simplest  form  growth  follows 
directly  from  assimilation,  from  the  incorporation  of 
new  material  which  increases  the  mass  of  the  protoplasm. 
Unless  this  new  protoplasm  is  destroyed  as  quickly 
as  or  more  quickly  than  it  is  formed,  increase  in  bulk 
must  result.  In  amoeba  the  animal  simply  increases  in 
size  till  it  reaches  a  certain  limit,  when  division  takes 
place,  and  the  two  daughter  amoebae  go  on  feeding  and 
growing  independently.  In  more  complex  organisms 
of  definite  shape  growth  takes  very  various  forms  in 
different  parts  of  the  body,  but  it  always  leads  to 
permanent  increase  in  the  bulk  of  the  organism — a 
temporary  inflation  of  part  of  the  body,  for  example, 
is  not  a  process  of  growth. 

The  higher  plants  differ  widely  from  the  higher 
animals,  in  showing  localised  growth,  in  most  cases 
at  the  ends  of  the  branches  of  the  shoot  and  of  the 
root,  leading  to  increase  in  length  of  the  branches, 
and  this  may  be  more  or  less  continuous  throughout 
1  Greek  dvd,  up.  »  Greek  /card,  down. 


8O  THE   VITAL  FUNCTIONS 

the  life  of  the  plant.  In  addition  to  this  increase  in 
length  of  the  branches,  definite  layers  of  cells  inside 
the  plant  may  also  grow,  increasing  the  thickness  of 
the  stem  and  root,  as  happens  in  the  case  of  trees. 

Most  animals  have  a  definite  growth  phase  in  youth, 
which  involves  the  whole  of  the  body,  but  is  different 
in  nature,  amount  and  duration  in  the  different  organs. 
Thus  a  child's  trunk  and  limbs  grow  more,  from  birth 
to  adult  life,  than  its  head,  and  similar  differences  are 
shown  in  the  growth  of  the  different  internal  organs. 

Both  in  animals  and  plants  it  is  clear  that  the  growth 
of  an  individual  living  cell  is  a  different  thing  from  the 
growth  of  the  organism  as  a  whole,  though  the  latter 
depends  upon  the  growth  and  division  of  the  cells. 

(4)  Differentiation. — A  multicellular  organism  arises, 
during  its  individual  lifetime,  from  a  single  cell.  This 
cell  divides,  and  the  daughter  cells,  instead  of  separating 
from  one  another,  as  they  do  after  cell  division  in. 
amoeba,  remain  together,  and  again  divide,  the  new 
daughter  cells  behaving  in  the  same  way.  In  this 
manner  the  adult  multicellular  body  is  produced.  But 
the  cells  of  this  body  are  not  all  alike  ;  sets  of  them  are 
grouped  together  into  tissues  and  organs,  and  the  cells 
of  one  tissue  differ  widely  from  those  of  another  in 
form,  structure  and  function.  The  process  of  becoming 
different  is  called  differentiation,  and  accompanies  the 
growth  of  the  bodies  of  all  multicellular  organisms. 
Thus  growth  depends  upon  the  two  processes,  cell 
division  and  cell  differentiation. 

The  protoplasm  of  a  single  cell  may  be  differentiated. 
Thus  in  theProtococcits  cell  ihechloroplast  is  differentiated 
from  the  colourless  cytoplasm,  whereas  in  amoeba 
there  is  no  such  differentiation.  In  some  unicellular 
animals  the  differentiation  reaches  a  high  pitch,  the 
structure  of  the  single  cell  being  very  complicated. 


RESPIRATION  8l 

But  such  intracellular  differentiation  r  has  limits,  because 
a  cell  is  unworkable  if  it  exceeds  a  certain  size,  and  the 
elaborate  differentiation  of  the  higher  organisms  depends 
upon  their  multicellular  structure. 

(5)  Respiration.  —  The  functions  hitherto  considered 
are  all  concerned  with  building  up  the  protoplasm  of 
the  organism.  We  have  now  to  consider  processes  con- 
nected with  the  expenditure  of  energy  by  the  organism 
and  with  the  breaking  down  of  organic  substances. 

When  the  amoeba  moves  it  does  work  and  spends 
energy  in  doing  it.  This  energy  is  derived  from  the 
energy  locked  up  (potential  energy)  in  the  molecules  of 
organic  substance  taken  in  as  food  —  largely  in  the 
carbohydrates.  The  molecules  of  these  substances  are 
broken  down  into  simpler  substances  and  largely 
oxidised  in  the  process,  the  energy  being  set  free  and 
appearing  in  the  kinetic  form,  i.e.  as  mass  motion  — 
for  instance,  the  streaming  of  the  amoeba  cytoplasm  — 
and  heat.  Free  oxygen  is  necessary  to  carry  out  this 
process,  and  that  is  why  free  oxygen  is  required  by 
very  nearly  all  living  beings.  Aquatic  organisms  get 
their  oxygen  from  that  which  is  dissolved  in  the  water 
they  live  in,  terrestrial  forms  direct  from  the  air.  In 
its  absence  the  activity  of  the  organism  comes  to  an  end. 

The  release  of  energy  is  obtained  almost  entirely  by 
the  oxidation  of  sugar  (glucose)  with  the  formation  of 
carbon  dioxide  and  water. 

The  generalised  equation  is  :  — 


C6H12O6  -}-  6O2  =  6CO2 

glucose        oxygen        carbon         water 

dioxide 
potential  energy  —  >  kinetic  energy 

(mass  motion  and  heat) 

This  process  is  what  is  called  in  biology  respiration. 

1  Differentiation  within  the  cell,  from  Latin  intra,  within. 

6 


82  THE   VITAL   FUNCTIONS 

The  same  chemical  process  (oxidation)  takes  place 
when  we  burn  sugar  in  the  air,  the  same  products, 
carbon  dioxide  and  water,  and  kinetic  energy  in  the 
form  of  heat,  resulting.  When  we  burn  coal  or  petrol 
in  an  engine  a  similar  process  takes  place.  Here  the 
kinetic  energy  is  partly  used  to  drive  a  machine,  for 
instance  a  steam  locomotive  or  a  motor  car,  and  this 
mass  motion  is  comparable  with  the  mass  motion  of 
protoplasm.  In  both  cases  some  of  the  energy  appears 
as  heat,  raising  the  temperature  of  the  cell  or  of  the 
engine,  as  the  case  may  be. 

The  living  protoplasm  of  all  plants  requires  free 
oxygen  for  respiration  just  like  that  of  animals,  though 
plants  do  not  use  it  so  quickly  as  active  animals  do. 
In  green  plants  in  light  respiration  is  masked,  because, 
as  we  shall  see  in  a  later  chapter,  the  opposite  process — 
the  formation  of  sugar  from  carbon  dioxide  and  water, 
with  production  of  free  oxygen — is  carried  out  at  a 
greater  rate  than  respiration.  In  plants  (and  in  parts 
of  plants)  that  are  not  green  and  are  growing  quickly, 
i.e.  expending  much  energy,  for  example  in  germinating 
seeds  and  opening  flower  buds,  a  great  quantity  of 
oxygen  is  used,  and  has  to  be  taken  from  the  air,  and 
a  great  deal  of  carbon  dioxide  and  heat  are  produced. 

"  Breathing "  in  the  higher  animals,  also  called 
"  respiration,"  *  consists  of  the  rhythmically  alternating 
processes  of  "  inspiration  "  and  "  expiration."  The 
former  is  the  taking  of  air  into  the  lungs  so  that  oxygen 
may  be  absorbed  through  the  walls  of  the  lung  tissue 
by  the  haemoglobin  of  the  red  blood  corpuscles,  and 
carried  by  them  in  the  blood  stream  to  all  the  tissues 
of  all  the  organs  of  the  body,  where  respiration  in  the 

1  This  of  course  is  the  original  meaning  of  the  term  (from  Latin 
spiro,  breathe). 


RESPIRATION,    KATABOLISM   AND   EXCRETION          83 

general  biological  sense  takes  place  in  the  living  cells. 
The  return  blood  stream  comes  back  to  the  lungs  laden 
with  carbon  dioxide,  the  result  of  the  tissue  respiration, 
and  this  gas,  diffusing  into  the  air  passages,  passes  out 
of  the  body  in  the  "  expired  "  air.  Here  again  we 
get  the  contrast  between  the  function  of  the  organism 
as  a  whole,  in  this  case  carried  out  by  special  organs — 
the  lungs — and  the  function  of  the  individual  cells  of 
which  it  is  composed.  It  is  especially  in  the  muscles 
that  sugar  is  most  actively  broken  down  in  the  bodies 
of  the  higher  animals,  and  accordingly  it  is  here  that 
the  greatest  amount  of  kinetic  energy  is  set  free,  as  we 
see  in  the  vigorous  contractions  of  our  muscles. 

Plants  have  no  "  organs  of  respiration  "  comparable 
with  the  lungs  of  the  higher  vertebrate  animals.  The 
oxygen  used  in  respiration  by  the  living  cells  of  plants 
diffuses  into  the  plant  from  the  air  through  the  system 
of  air  spaces  between  the  cells  (intercellular  spaces), 
and  so  through  the  water  saturating  the  wet  cell  walls 
into  the  protoplasm.  The  oxygen  actually  used  by  the 
living  cells  in  respiration,  both  in  animals  and  plants, 
is  always  oxygen  dissolved  in  liquid,  though  it  is 
ultimately  derived  from  the  air. 

(6)  Katabolism  and  (7)  Excretion. — Respiration  is 
essentially  a  katabolic  process,  involving  the  breaking 
down  of  an  organic  substance  (sugar)  in  the  living  cell. 
But  animals  have,  in  addition,  a  nitrogenous  katabolism 
involving  the  breaking  down  of  proteins.  This  process 
goes  on  largely,  though  not  wholly,  in  the  liver,  and  the 
comparatively  simple  nitrogenous  substances  formed  in 
the  liver  pass  into  the  blood  and  are  excreted  through 
the  kidneys  as  urea  (CH4N2O)  and  uric  acid  (C5H4N4O3), 
and  pass  out  of  the  body  in  the  urine.  In  the  amoeba 
these  nitrogenous  excretions  are  probably  expelled 


84  THE   VITAL   FUNCTIONS 

through  the  contractile  vacuole.  It  will  be  noted 
that  these  substances  are  not  nearly  so  fully  oxidised 
as  CO2,  i.e.  oxygen  forms  a  much  smaller  proportion  of 
the  molecule,  and  the  animal  derives  a  negligible 
quantity  of  its  energy  from  the  breaking  down  of 
proteins.  Plants  have  no  comparable  nitrogenous  kata- 
bolism.  If  katabolites  are  formed  from  the  breaking 
down  of  proteins  they  are  not  systematically  excreted, 
but  are  probably  often  used  again  for  the  formation  of 
proteins.  Plants  are,  in  fact,  much  more  economical  of 
their  nitrogen  than  are  animals.  Various  substances, 
some  of  them  containing  nitrogen,  may  be  cast  off 
by  plants,  as  in  the  bark  of  trees,  while  carbon  dioxide 
regularly  diffuses  out  of  the  tissues  that  are  not  green, 
and  from  all  tissues  in  darkness,  but  there  is  in  plants 
no  specialised  excretory  system. 

(8)  Movement. — This  is  the  most  obvious  of  all  the 
expressions  of  life.  We  can  again  distinguish  between 
the  movement  of  the  protoplasm  of  living  cells  such  as 
we  see  in  the  streaming  of  the  protoplasm  in  amoeba 
and  the  movement  of  parts  (for  instance  the  limbs, 
involving  whole  organs  and  systems  of  tissues  in  the 
higher  animals),  or  the  movement  of  the  whole  organism 
from  place  to  place  (locomotion).  In  amoeba  we  can 
observe  under  the  microscope  how  the  first  leads  directly 
to  the  second  (formation  of  pseudopodia),  and  this 
again  to  the  third.  In  a  higher  animal  the  same  con- 
nexion occurs,  but  the  chain  of  causation  is  much  longer. 
The  movements  of  parts,  for  instance  the  limbs,  depend 
on  the  contraction  of  the  living  substance  of  highly 
specialised  tissues,  the  muscles  ;  through  the  definite 
relationship  of  these  with  the  bones  the  movements 
of  several  muscles,  co-ordinated  through  the  agency  of 
the  central  nervous  system,  leads  to  the  movement  of 


MOVEMENT   OF   PROTOPLASM  85 

a  limb ;  while  the  co-ordinated  movements  of  the 
limbs  may  result  in  the  movement  of  the  body  from 
place  to  place. 

In  the  cells  of  plants  the  rapid  streaming  movement 
of  the  cytoplasm,  which  in  certain  cases  may  be  observed 
under  the  microscope  to  stream  round  the  cell,  is  perhaps 
always  the  result  of  an  injury  or  other  shock  :  there  is 
no  evidence  that  it  occurs  normally  in  the  untouched 
plant.  Nevertheless  it  provides  conspicuous  proof  of 
the  power  of  plant  protoplasm  to  move  comparatively 
rapidly.  There  is  also  plenty  of  evidence  of  the  power 
of  rapid  movement  of  plant  cells  under  normal  conditions, 
e.g.  in  the  motility  of  many  of  the  lower  unicellular 
plants  and  of  the  reproductive  cells  in  many  fixed 
plants,  though  these  move  by  means  of  the  contraction 
of  special  delicate  cytoplasmic  processes  projecting  from 
the  cell  body  (cilia  and  flagella)  whose  rapid  beating 
pulls  or  pushes  the  organism  through  the  surrounding 
liquid.  Many  unicellular  animals  also  move  in  this  way. 

The  commonest  form  of  movement  of  the  protoplasm 
of  plant  celjs  is,  however,  a  slow  streaming  movement, 
usually  too  slow  to  observe  directly,  but  evidenced  by 
the  frequent  changes  in  place  of  the  constituents  of  the 
living  cell  body,  such  as  the  nucleus,  and  in  green  cells, 
the  chloroplasts.  The  growth  of  a  plant  clearly  involves 
the  result  of  a  multitude  of  slow  movements,  and  all 
of  these,  like  the  much  quicker  movements  of  animals, 
depend  upon  a  constant  supply  of  energy  which  is 
derived  from  the  release  of  potential  energy  by  the 
breaking  down  of  organic  molecules  through  oxidation. 
A  moment's  reflection  will  convince  us  of  the  large 
amount  of  energy  involved  in  the  building  up  and 
raising  of  the  branches  of  a  tree,  eventually  weighing 
perhaps  many  tons,  high  into  the  air  against  the  force 


86  THE   VITAL   FUNCTIONS 

of  gravity,  and  in  the  burrowing  of  the  roots  far  into 
the  substance  of  resistant  soil. 

(9)  Response  to  Stimuli. — All  living  protoplasm  responds 
to  various  stimuli  in  various  ways,  or  in  other  words  it 
is  sensitive  to  the  forces  acting  upon  it  from  its  sur- 
roundings. This  response  depends  upon  the  setting 
up  of  processes  within  the  protoplasm  which  result  in 
some  change  in  the  activity  of  the  latter.  The  most 
conspicuous  expression  of  response  to  stimulus  is  a 
movement.  It  is  necessary  to  be  clear  as  to  the  differ- 
ence between  this  kind  of  movement  and  the  merely 
passive  movement  which  results  from  the  application 
of  an  external  force.  For  instance,  if  one  man  pushes 
another  over  the  edge  of  a  cliff,  the  falling  of  the  victim 
is  a  passive  movement,  resulting  from  the  force  of  the 
push  and  the  force  of  gravity.  But  if  the  intended 
victim,  after  receiving  the  push,  saves  himself  by  a 
sudden  leap  to  one  side,  that  movement  is  a  response 
to  the  stimulus  of  the  push.  It  involves  a  change  in 
the  activity  of  his  nerve  and  muscle  cells.  This  is  a 
very  complicated  response  in  a  highly  complex  organ- 
ism, though  it  is  not  nearly  so  complicated  as  the 
responses  we  carry  out  every  minute  in  the  course  of 
our  daily  lives. 

The  very  simplest  organisms,  however,  exhibit  re- 
sponses to  various  stimuli,  and  these  are  specific,  i.e. 
they  vary  quite  definitely  according  to  the  stimulus. 
It  is  probable  that  they  can  all  be  explained  as  con- 
sisting of  a  series  of  purely  physical  and  chemical  pro- 
cesses within  the  protoplasm,  set  in  motion  by  the 
stimulus ;  as  for  instance  in  the  case  of  the  ingestion 
of  food  by  the  protoplasm  of  the  amoeba,  which,  as  we 
saw,  is  closely  paralleled  by  the  ingestion  and  ejection 
of  a  glass  splinter  coated  with  shellac  by  a  drop  of 


TROPISMS.      REPRODUCTION  87 

chloroform.  But  in  most  cases  we  do  not  as  yet  know 
nearly  enough  of  the  chemistry  and  physics  of  the  living 
cell  to  furnish  a  detailed  explanation  of  the  causes  and 
course  of  the  particular  response  in  every  case. 

When  the  whole  organism  moves  in  response  to  an 
external  stimulus,  the  motion  is  called  a  taxis.1  When, 
as  in  a  fixed  plant,  an  organ  bends  in  response  to  a 
stimulus,  the  motion  is  called  a  tropism.2  For  instance, 
motile  unicellular  green  plants  (e.g.  Chlamydomonas, 
Chapter  XII)  are  positively  phototactici  in  light  of 
weak  or  medium  intensity  coming  from  one  side 
i.e.  they  move  towards  the  source  of  such  light.  Motile 
unicellular  organisms  in  general  are  chemotactic,  either 
positively  or  negatively,  i.e.  they  move  towards  or  away 
from  the  source  of  various  chemical  substances  diffusing 
from  a  given  source,  according  to  the  nature  of  the 
substance.  The  tips  of  the  branches  of  the  stem  and 
root  of  a  higher  plant  are  phototropic,  geotropic*  and 
hydrotropic  5  in  different  cases,  bending  towards  or  away 
from  a  source  of  light,  the  direction  in  which  gravity 
is  acting,  or  a  source  of  moisture.  Thus  the  tips  of 
roots  are  positively  geotropic  and  hydrotropic,  but 
negatively  pjiototropic,  and  so  on.  Of  the  detailed 
effects  of  other  stimuli,  e.g.  electrical  stimuli,  on  the 
protoplasm  of  cells  we  do  not  know  enough  to  speak 
confidently,  though  we  know  that  such  effects  exist. 

(10)  Reproduction  may  be  defined  as  the  production 
of  a  new  individual  or  new  individuals  from  pre-existing 
ones.  It  has  been  usefully  defined  as  discontinuous 
growth,  since  the  formation  of  a  new  individual  always 
involves  processes  of  growth,  and  this  growth  is 
discontinuous,  for  it  begins  in  the  parent  organism 

1  Greek  Ta£tg,  a  disposition.    '  z  Greek  rpo-rroq,  a  turn. 

3  Greek  0cog,  ^COTOJ,  light.         4  yfj,  the  earth.        5  vdcop,  water. 


88  THE  VITAL  FUNCTIONS 

and  is  carried  on  in  the  separate  new  organism,  or 
organisms,  which  is,  or  are,  the  offspring. 

The  simplest  form  of  reproduction  is  binary  fission 
such  as  we  saw  in  the  amoeba,  the  division  of  the  body 
of  the  parent  into  the  bodies  of  the  offspring.  We 
do  not  completely  understand  the  forces  which  set  this 
process  at  work.  All  we  can  say  is  that  it  seems  to 
be  ultimately  determined  by  a  size  limit  corresponding 
with  a  limit  of  the  surface/bulk  ratio,  which  decreases 
as  the  organism  grows  in  size.  We  know  that  the 
surface  of  a  sphere  increases  as  the  square  of  the  radius, 
while  its  volume  increases  as  the  cube  of  the  radius. 
Thus  a  unicellular  organism,  as  it  grows  in  size,  will 
have  a  smaller  and  smaller  surface  in  proportion  to 
its  volume,  and  this  will  have  an  important  effect  on 
the  processes  going  on  within  the  protoplasm.  The 
supply  of  oxygen,  for  instance,  which  has  to  pass  through 
the  surface  from  the  surrounding  water  will  be  pro- 
gressively less  for  each  unit  volume  of  protoplasm 
the  larger  the  organism  becomes.  It  is  probable  that 
some  effect  of  this  nature  initiates  the  process  of  division. 

A  large  number  of  unicellular  organisms  are  reproduced 
by  binary  fission  or  by  some  simple  modification  of  it. 
But  multicellular  organisms  produce  special  reproduc- 
tive cells  which  are  separated  from  the  bodies  of  the 
parents  and  produce  new  organisms  (offspring)  by 
growth  and  differentiation,  which  may  begin  and  be 
carried  on  for  some  distance  (in  the  highest  organisms 
to  the  greatest  extent)  before  complete  separation 
occurs.  In  Chapter  XII  we  shall  consider  the  relation 
of  these  special  reproductive  cells  to  the  individual 
unicellular  organism  which  reproduces  itself  by  binary 
fission. 

Reproduction  may  also  take  place,  as  in  most  plants 


PRACTICAL   WORK  89 

and  in  some  of  the  lower  multicellular  animals,  by  the 
separation  of  parts  of  the  body  which  are  not  special 
reproductive  cells,  e.g.  by  the  separation  of  some 
segments  of  the  body  in  worms  or  by  the  rooting  of 
cuttings  and  "  layers  "  of  plants.  This  formation  of 
new  individuals  from  non-specialised  or  only  slightly 
specialised  parts  of  the  ordinary  body  of  the  organism 
is  called  vegetative  reproduction.  It  does  not  occur  in 
the  highest  animals. 

PRACTICAL   WORK. 

A.  RESPIRATION  AND  PRODUCTION  OF  HEAT  IN  LIVING  SEEDS 
AND  FLOWER  BUDS. 

(1)  The    test    tube    provided    contains    living    barley    grains 
(seeds)   which  have  been  wetted  and  placed  in  the  test  tube, 
which  was  corked  on  the  previous  day.     They  absorb  water  and 
swell  as  a  preliminary  to  sprouting.     They  also  absorb  oxygen 
and  give  off  carbon  dioxide  in  considerable  quantity  as  a  result 
of  the  rapid  respiration  of  their  living  cells,  which  resume  active 
life  as  a  result  of  absorption  of   water    and  oxygen.     Test    for 
the  carbon  dioxide  by  dipping  the  rod  into  lime  water,  carefully 
uncorking  the  test  tube  and  inserting  the  tip  of  the  rod  close  to 
the  grains  without  touching  them.     After  thirty  seconds  or  so  look 
at  the  drop  of  lime  water  on  the  rod  against  a  dark  background. 

(2)  Examine  the  demonstration  consisting  of  a  U-tube  con- 
taining coloured  water  and  connecting  with  two  corked  flasks, 
one  containing  wetted  living  and  the  other  wetted  dead  barley 
grains.     Fixed  to  the  cork  inside  each  flask  is  a  lump  of  caustic 
potash  which  readily  absorbs  carbon  dioxide.     In  the  flask  con- 
taining living  grains  oxygen  has  been  absorbed  by  the  grains 
while  the  corresponding  carbon  dioxide  produced  is  absorbed 
by  the  caustic  potash.     In  the  flask  containing  dead  grains  no 
such   change   has   occurred.     Hence   the   total   amount  of   free 
gas  in  the  two  flasks  changes  and  the  pressure  on  the  surface 
of  the  liquid  in  the  two  limbs  of  the  U-tube  becomes  unequal. 

(3)  Examine  the  demonstration  showing  that  the  temperature 
rises  in  the  middle  of  a  mass  of  wetted  living  barley  grains  and 
not  in  the  corresponding  mass  of  dead  ones. 

(4)  Examine  the  demonstration  showing  the  rise  of  tempera- 


90  THE   VITAL  FUNCTIONS 

ture  of  chrysanthemum  (or  other  suitable)  buds  in  a  vacuum 
(thermos)  flask.  Here  the  evolution  of  heat  is  "  cumulative  " 
for  a  time  because  the  heat  cannot  escape,  and  the  rising 
temperature  accelerates  the  process  of  respiration. 

B.  LOCALISATION  OF  GROWTH  IN  PLANTS.     RESPONSE  TO 

STIMULUS. 

(1)  Take  two  beans  which  have  germinated  and  produced 
roots  from  one  to  two  inches  long.     If  the  surface  of  the  root  is 
wet,  dry  it  carefully  with  a  piece  of  torn  filter  paper,  taking  great 
care  not  to  break  or  injure  the  root.     With  a  carefully  sharpened 
stick  (or  lead  pencil)  dipped  in  Indian  ink  make  marks  on  the 
root  at  intervals  of  one-tenth  of  an  inch  from  the  top  to  the  seed. 
Now  push  a  large  pin  carefully  through  each  bean,  and  fasten 
to  the  under  side  of  the  cork  of  a  wide-mouthed  bottle,  so  that 
when  the  cork  is  replaced  the  root  of  one  points  vertically  down- 
wards while  that  of  the  other  is  horizontal.     Pour  a  little  water 
into  the  bottom  of  the  bottle,  replace  the  cork  with  the  beans 
pinned  to  it  and  turn  the  bottle  upside  down  for  a  few  seconds, 
so  as  to  wet  the  beans  and  provide  water  for  their  growth.     Leave 
the  bottles  till  next  time. 

(2)  Examine  the  demonstration  showing  the  aggregation  of 
the  green  motile  unicellular  plants  (Euglena)  on  the  glass  side  of 
a  jar.     The  Euglenae  were  scattered  through  the  water  in  the 
jar,  which  was  covered  with  black  paper.     The  only  light  reaching 
the  Euglenae  came  through  the  stencil  holes  in  the  paper,  and 
they  moved  towards  it,  eventually  adhering  to  the  glass  under 
the  openings. 

C.  ORGANIC  FOODSTUFFS  OF  PLANTS  AND  SOME  OF  THEIR 

COLOUR  REACTIONS. 

Representatives  of  the  three  great  classes  of  organic  food- 
stuffs— proteins,  carbohydrates  and  fats — are  stored  in  seeds  and 
other  storage  organs  such  as  tubers,  and  are  used  to  feed  the 
young  plant  arising  from  the  seed  or  tuber  before  it  is  able  to 
feed  itself.  These  foodstuffs  can  be  identified  under  the  micro- 
scope, partly  by  their  appearance,  partly  by  the  use  of  different 
stains  and  reagents  : — 

(i)  With  watery  solution  of  iodine  in  potassium  iodide 
(commonly  called  "  iodine  solution  ")  grains  of  proteins 
turn  yellow  or  yellow  brown,  starch  blue,  cellulose 
and  fats  remain  uncoloured. 


PRACTICAL   WORK  QI 

(2)  With    solution    of  zinc   chloride     in    potassium    iodide 

("  Schulze's  solution  ")  proteins  turn  yellow  as  before, 
starch  blue,  cellulose  blue  or  purple,  fats  remain 
uncoloured. 

(3)  With  Sudan  3  (an  anilin  dye)  proteins,  starch  and  fats 

remain  uncoloured,  fats  turn  orange-red. 

Stain  an  example  of  each  of  the  three  sections  of  different 
seeds  :  (a)  bean,  (b)  castor  oil  plant  (Ricinus),  (c)  lupin,  in  watch- 
glasses  containing  the  reagents  (i),  (2),  (3)  above.  Wash  in 
water  and  mount  in  dilute  glycerine.  Draw  a  single  suitable 
cell  of  each  section  under  the  high  power,  and  determine  the 
classes  of  food  substance  present  in  each. 


CHAPTER     VI 
THE   CELL 

WE  have  now  become  acquainted  with  two  simple 
living  organisms,  Amoeba  and  Protococcus,  with  their 
structure,  behaviour  and  life  histories  ;  and  we  have 
seen  that  the  "  vital  functions  "  which  they  exhibit  are 
expressions  of  the  activity  of  the  protoplasm  of  which 
their  bodies  are  composed,  and  that  these  same  "  vital 
functions  "  are  exhibited  by  all  living  organisms,  however 
large  and  complex,  however  different  in  structure  and 
appearance — by  trees  and  human  beings,  just  as  by 
Protococcus  and  Amoeba.  But  we  have  also  seen  that 
the  exact  ways  in  which  the  various  functions  are 
performed  by  the  organism  as  a  whole  varies  with  its 
structure,  and  before  we  can  understand  these  variations 
in  any  detail  we  must  become  acquainted  with  the 
organisation  and  variations  of  the  unit  of  which  organic 
structure  is  built  up — the  cell. 

Unicellular  and  Multicellular  Organisms.  Differen- 
tiation and  Division  of  Labour.  Tissues. — We  have 
already  seen  that  the  whole  body  of  an  amoeba  is  a 
unit  of  living  protoplasm  consisting  of  cytoplasm  and 
nucleus  :  the  same  is  true  of  Protococcus,  which  has, 
however,  a  special  green  protoplasmic  body  called  the 
chloroplast  within  the  cell  which  enables  the  organism 
to  make  carbohydrate  out  of  carbon  dioxide  and  water, 
and  also  a  non-living  cell  wall  of  cellulose  (a  complex 
carbohydrate)  which  is  formed  by  the  protoplasm. 


DIFFERENTIATION  93 

These  organisms  are  called  unicellular,  because  they 
consist  of  one  such  unit,  in  contrast  with  the  great 
majority  of  plants  and  animals  which  are  composed 
of  many  such  units,  each  with  its  own  cytoplasm  and 
nucleus — which  are  in  fact  multicellular, 

The  unicellular  organism  may  show  a  considerable 
differentiation  of  parts  within  the  cell.  The  primary 
differentiation  into  nucleus  and  cytoplasm  is  found  in 
all  (except,  perhaps,  the  bacteria,  see  Chapter  IX), 
but  in  Protococcus  we  find  the  differentiation,  charac- 
teristic of  all  green  plant  cells,  of  the  cytoplasm  into 
the  chloroplast  and  colourless  cytoplasm.  Many  of 
the  higher  Protozoa  have  quite  complicated  cells.  For 
instance  the  cell  may  have  a  firm  cortical  cytoplasm 
(gel  structure)  which  maintains  the  shape  of  the  body, 
a  definite  opening  (mouth  and  gullet)  through  this  leading 
into  the  central  more  motile  cytoplasm  (sol),  as  well 
as  numerous  exceedingly  fine  threads  of  cytoplasm 
(cilia)  which  project  from  the  surface  of  the  body,  and 
by  their  rapid  beating  pull  the  animal  quickly  through 
the  water.  Thus  we  have  different  parts  of  the  cell, 
differentiated  and  performing  different  functions  for  the 
whole,  e.g.  the  mouth  and  gullet  for  feeding,  the 
cilia  for  motion,  etc.  Other  kinds  of  Protozoa  have 
skeletons  of  non-living  substance,  enclosing  the 
protoplasm  which  projects  through  openings  in  the 
skeleton. 

Though  this  differentiation  is  often  carried  much 
further  in  a  unicellular  organism  like  a  Protozoon  than 
in  the  individual  cells  of  a  multicellular  organism,  the 
differentiation  between  different  cells  which  is  possible 
in  a  multicellular  organism  is  much  greater,  because 
there  is  no  limit  to  the  size  of  the  organism.  In  the 
simpler  multicellular  plants  (e.g.  Green  Algae)  there  is 


94  THE   CELL 

little  or  no  differentiation  between  different  cells,  but 
in  the  higher  plants  it  is  considerable. 

A  set  of  similar  cells  which  perform  one  special 
function  or  set  of  functions  in  the  organism  is  called  a 
tissue.1  Thus  in  the  higher  plants  there  are  absorbing 
tissues,  conducting  tissues,  sugar-forming  tissues,  pro- 
tective tissues,  etc.  Specialisation  of  different  cells  for 
different  functions  reaches  a  far  higher  pitch  in  the. 
higher  animals,  where  different  tissues  are  associated 
in  the  different  organs,  and  work  together  to  perform 
the  special  function  of  the  organ,  e.g.  lungs,  heart, 
liver,  kidneys,  etc. 

However  specialised  the  structure  and  work  of  a 
cell  may  be,  it  must  necessarily  be  able  to  carry  out 
the  vital  functions  described  in  the  last  chapter,  or  it 
would  not  be  alive.  It  depends,  however,  upon  its 
position  in  the  body  and  its  relation  to  other  cells  and 
tissues  for  the  essentials  of  its  continued  existence — 
it  cannot  lead  a  free  independent  life  like  the  cell  of 
a  Protozoon  and  obtain  its  food  and  oxygen  directly 
from  outside  the  organism.  Thus  the  muscle  cells 
depend  for  their  food  and  oxygen  on  the  circulation 
of  the  blood  which  carries  these  necessary  substances 
to  them.  Each  highly  differentiated  cell  or  tissue 
specialises  on  one  function  of  protoplasm,  which  repre- 
sents its  particular  work  for  the  organism  as  a  whole. 
Thus  the  fibres  of  the  muscle  tissue  (Fig.  5,  F)  show 
the  power  of  contracting — a  specialised  form  of  move- 
ment— in  a  very  high  degree,  and  by  means  of  the 
co-ordination  of  the  contractions  of  many  muscles  the 
organism  moves.  The  cells  of  the  glands  attached  to  the 
alimentary  canal  specialise  in  the  production  of  enzymes 
which  they  pour  out  into  the  cavity  of  the  gut  for 
1  From  the  analogy  of  its  texture  with  that  of  a  textile  fabric. 


SPECIALISATION   OF   CELLS 


95 


purposes  of  digestion  of  the  food  of  the  whole  organism. 
Some  of  the  gland  cells  lining  the  air  passages  are 
ciliated  and  others  produce  mucus  (Fig.  5,  B). 


FIG.  5. — Cells  from  different  tissues  of  a  higher  vertebrate  animal. 
A,  blood  corpuscles ;  r,  red  corpuscles — these  are  not  complete 
cells  and  have  no  nuclei ;  w,  white  corpuscle,  whose  structure 
and  movements  are  very  similar  to  those  of  amoeba.  B,  cells 
of  the  ciliated  glandular  epithelium  lining  the  trachea  (air  tube 
to  lungs) ;  n,  nucleus ;  m,  mucus  being  expelled  from  the  cell ; 
c,  cilia.  C,  cells  of  cartilage  recently  divided,  embedded  in 
cartilaginous  matrix  secreted  by  the  cells ;  n,  nucleus.  D,  cells 
and  fibres  of  connective  tissue  (which  forms  the  "  packing  "  of  the 
organs  of  the  body) ;  n,  nucleus.  E,  fat-forming  cells  ;  /,  globules 
of  fat;  n,  nucleus,  F,  Part  of  striped  muscle  fibre.  Here  the 
protoplasm  is  highly  modified  into  special  contractile  substance ; 
n,  nucleus :  many  nuclei  in  each  fibre,  which  is  composed  of  many 
fused  cells.  The  nuclei  are  situated  just  below  the  very  thin 
outer  membrane. 


96  THE   CELL 

The  white  blood  corpuscles,  on  the  other  hand,  are 
unspecialised  cells,  very  much  like  amoebae  (Fig.  5, 
A,  w).  Highly  specialised  cells  lose  the  power  of 
multiplying  by  division  as  soon  as  the  tissue  to  which 
they  belong  is  fully  differentiated.  In  general  the 
power  of  reproduction  by  division  is  the  function  of 
an  unspecialised  cell. 

The  Cell  Doctrine. — Every  multicellular  organism  begins 
life  as  a  single  cell,  which  is  separated  from  the  body 
of  the  parent  and  by  cell  division,  growth,  and  (in  all 
but  the  simplest  forms)  by  differentiation  of  the  products 
of  division  gives  rise  to  the  body  of  the  adult  offspring. 
In  this  process  of  cell  division  the  daughter  cells  do  not 
separate  from  one  another,  as  in  the  binary  fission 
of  a  unicellular  organism,  but  remain  together,  and  in 
all  the  higher  forms  undergo  differentiation  into  tissues 
and  organs  to  form  the  different  parts  of  the  body. 

This  is  the  basis  of  the  generalisation — known  as  the 
cell  doctrine — that  organisms  consist  of  cells,  which  are 
the  structural  units  of  the  organism,  and  that  the 
functions  of  the  organism  as  a  whole  are  made  up  of 
the  sum  of  the  functions  of  all  its  cells.  Broadly 
speaking  this  is  true,  but  certain  qualifications  have  to 
be  made. 

In  the  first  place,  in  some  of  the  lower  forms  the 
body  does  not  consist  of  distinct  cells,  though  it  contains 
a  great  many  nuclei.  Thus  in  some  algae  and  fungi 
among  the  plants  the  body  is  composed  of  a  branched 
tube  of  cellulose  (or  similar  substance)  enclosing  a 
continuous  mass  of  cytoplasm  in  which  are  scattered 
numerous  nuclei.  As  the  plant  feeds  and  grows  the 
protoplasm  at  the  tips  of  the  branches  increases  and 
the  nuclei  embedded  in  it  increase  in  number  by  division, 
the  wall  covering  the  tip  continually  having  new 


THE   CELL   DOCTRINE  97 

layers  added  to  it  by  the  protoplasm  within,  and  being 
pushed  out  as  the  tube  grows  in  length.  Here  there 
is  no  cell  structure,  and  such  plants  are  called  non- 
cellular.  Also  there  are  parts  of  animals,  and  phases 
in  the  development  of  animals,  where  there  is  no  clear 
cell  structure,  but  for  instance  a  network  of  cytoplasm 
with  a  nucleus  at  each  "  node  "  of  the  network,  i.e. 
at  each  thickening  of  the  cytoplasm  where  several 
branches  of  the  network  meet.  This  kind  of  structure 
does  not  however  differ  essentially  so  far  as  its  working 
is  concerned  from  ordinary  cell  structure.  We  have 
already  seen  (p.  66)  that  the  nucleus  is  essential  to  the 
carrying  out  of  the  vital  functions  of  the  protoplasm  ; 
and  in  these  cases  of  non-cellular  or  incomplete  cellular 
structure  we  must  suppose  that  each  nucleus  governs, 
so  to  speak,  a  certain  sphere  of  cytoplasm  around 
it,  though  this  sphere  is  not  sharply  separated,  as  it 
is  in  ordinary  cellular  structure,  from  the  neighbouring 
spheres  or  units.  Such  a  working  unit  of  nucleus  and 
cytoplasm  has  been  called  an  energid.  An  ordinary 
living  cell  represents  one  such  energid,  but  in  non- 
cellular  living  structures  the  energids  are  not  separated 
by  clear  boundaries. 

The  second  qualification  of  the  cell  doctrine  that 
has  to  be  made  relates  to  the  unity  or  individuality  of 
the  multicellular  organism.  To  some  degree  in  all  such 
organisms,  and  very  notably  in  the  higher  animals 
with  a  central  nervous  system,  the  activities  of  each 
cell  are  modified  by  its  connexion  with  the  other  cells 
of  the  body,  so  that  the  organism  works  as  a  whole.  In 
this  sense,  as  we  saw  in  the  last  chapter,  the  organism 
as  a  whole  has  functions  which  are  something  distinct 
from  the  functions  of  the  individual  cells  or  energids 
of  which  it  is  composed,  though  these  are  built  up  as 
7 


98  THE   CELL 

a  result  of  the  sum  total  of  the  functions  of  all  its  cells 
or  energids. 

Cell  Walls  and  Intercellular  Substance.— In  animals 
the  substance  of  the  body  does  not  necessarily  consist 
entirely  of  the  actual  bodies  of  living  cells,  though  the 
whole  of  the  substance  is  formed  by  the  activities  of 
these.  The  cells  themselves  frequently  lie  in  a 
"  matrix  "  of  intercellular  substance  which  has  been 
formed  by  the  cells  which  were  originally  close  together 
but  eventually  lie  singly  or  in  groups  scattered  in  this 
matrix.  This  is  very  clearly  seen  in  the  case  of  cartilage 
(Fig.  5,  C)  and  bone  (which  is  developed  from  cartilage 
by  changes  in  the  matrix).  In  the  case  of  connective 
tissue,  the  tissue  which  forms  the  "  packing  "  of  the 
organs  of  the  body,  the  matrix  is  differentiated  into 
two  kinds  of  fibres  which  run  through  it,  singly  or  in 
bundles,  independently  of  the  connective  tissue  cells 
(Fig.  5,  D).  In  the  case  of  muscle  the  actual  cytoplasm 
of  the  original  cells  is  modified  into  a  peculiar  substance 
which  shows  no  cell  structure,  but  is  composed  of  fibres 
that  have  an  enormous  power  of  contractility.  The 
cell  nuclei  still  remain  lying  below  the  membrane 
enclosing  the  muscle  fibre  (Fig.  5,  F).  Some  animal 
tissues  have  a  distinct  cell  wall  surrounding  the  cyto- 
plasm of  each  cell.  This  may  be  composed  of  modified 
cytoplasm  or  of  simpler  organic  substances,  usually 
containing  nitrogen. 

Plants,  on  the  other  hand,  very  rarely  have  "inter- 
cellular substance."  So  much  of  the  substance  of  the 
plant  body  as  is  not  actually  composed  of  the  living 
bodies  of  cells  is  made  up  of  actual  cell  walls,  which 
in  the  adult  state  are  still  clearly  seen  to  have  been 
formed  by  the  cells,  and  the  basis  of  these  walls  is 
generally  cellulose.  The  cell  walls  of  a  plant  form  the 


THE   PLANT   CELL  99 

skeleton  or  framework  of  the  plant  body,  and  in  the 
higher  plants  a  considerable  part  of  the  structure  (in 
trees  the  great  bulk  of  the  structure)  is  composed  of 
the  walls  of  cells  which  have  died,  so  that  only  the 
wall  is  left.  These  walls  are  often  very  thick  and 
hard,  growth  in  thickness  and  the  deposition  of  sub- 
stances other  than  cellulose  having  taken  place  before 
the  protoplasm  of  the  cell  died  and  disappeared. 
Another  feature  of  the  tissues  of  the  higher  plants 
(land  plants)  is  the  existence  of  a  system  of  air  spaces 
(intercellular  spaces)  between  the  cells.  These  are 
made  by  the  separation  of  the  walls  of  certain  adjacent 
cells  from  one  another,  and  penetrate  the  plant,  com- 
municating with  the  outer  air  and  forming  the  channels 
by  which  gases,  such  as  water  vapour,  carbon  dioxide 
and  oxygen,  produced  or  used  by  the  living  cells,  diffuse 
from  these  to  the  air  outside,  or  vice  versd. 

THE  PLANT  CELL. 

The  first  cells  of  organisms  to  be  described  were  those 
of  the  cork  tissue  of  plants,  which  were  seen  by  the 
Englishman  Hooke  in  1667  with  the  compound  micro- 
scope that  was  devised  about  the  middle  of  the  seven- 
teenth century.  Hooke  compared  these  cork  cells  with 
the  "  cells "  of  honeycomb.  Thus  it  was  the  walls 
enclosing  cavities  to  which  the  term  was  first  applied. 
Little  accurate  knowledge  of  the  contents  of  these  cavities 
was  obtained  for  nearly  two  centuries.  But  in  1831 
the  English  botanist,  Robert  Brown,  discovered  the 
cell  nucleus,  and  in  1846  the  German  botanist,  von 
Mohl,  identified  the  semi-transparent  viscous  substance 
formed  within  many  of  the  "  cells  "  of  the  adult  higher 
plant  as  the  actual  living  substance  of  the  organism, 


100 


THE   CELL 


FIG.  6. — A,  embryonic  cells  from  the  meristem  (growing  point)  of 
the  root.  B,  beginning  of  vacuolation.  C,  cells  from  elonga- 
ting region  with  larger  vacuoles ;  c.w.,  cell  wall ;  cyt.,  cytoplasm  ; 
n,  nucleus ;  no.,  nucleolus ;  v,  vacuole.  The  top  right-hand  cell 
in  C  has  been  cut  and  water  admitted  to  the  cavity ;  the  nucleus 
has  absorbed  water  and  burst,  a  new  gel  membrane  (m)  being 
formed  on  the  surface  of  the  escaped  contents  in  contact  with 
the  water,  x  660  (after  Sachs). 


STRUCTURE  OF  PLANT  CELL          IOI 

and  called  it  protoplasm  J  This  term  was  then  extended 
to  the  essentially  similar  substance  found  in  animals. 
The  parallelism  between  the  units  of  protoplasm,  each 
with  its  nucleus,  in  animals  and  plants  was  soon  recog- 
nised, and  the  term  cell  gradually  came  to  be  used  in 
its  modern  sense  as  applying  primarily  to  these  living 
units,  which  may  or  may  not  have  definite  cell  walls. 
In  plants,  however,  as  we  have  seen,  they  have  walls 
in  the  vast  majority  of  cases,  and  the  term  cell  is  still 
applied  not  only  to  the  living  cytoplast-nucleus  unit, 
but  also  to  the  wall  and  cavity  after  the  protoplasm 
has  disappeared. 

The  adult  living  plant  cell  is  generally  characterised 
not  only  by  the  possession  of  a  cell  wall,  but  also  by 
a  large  central  vacuole  or  space  filled  with  a  watery 
liquid  (cell  sap),  enclosed  within  the  cytoplasm  (Fig.  6,  C). 
So  large  is  this  in  proportion  to  the  whole  space  within 
the  cell  that  the  cytoplasm  is  often  a  mere  thin  layer 
lining  the  wall.  The  nucleus,  which  always  remains 
surrounded  by  cytoplasm,  is  embedded  in  this  film 
and  bulges  it  out  into  the  vacuole  (Fig.  6,  C).  Vacuoles 
are  by  no  means  the  monopoly  of  plant  cells — they 
frequently  exist  in  those  of  animals — but  animal  cells 
as  a  class  are  not  characterised  by  the  possession  of 
a  large  central  vacuole,  the  formation  of  which  depends 
on  the  mode  of  nutrition  and  growth  of  the  typical 
plant  cell.  The  layer  of  cytoplasm  next  the  vacuole 
"  vacuole  wall  "),  like  the  external  layer  (ectoplasm  2) 
next  the  cell  wall,  is  an  exceedingly  thin  film — a  gel 
membrane,  such  as  we  saw  is  usually  formed  at  the 
boundary  of  a  colloid  sol  (Fig.  7). 

Embryonic  (Meristematic)  Cells.— In  the  higher  plants 

1  Greek  -nptaroq,  first,  and  -nXdo^a,  thing  formed   or  moulded,   as 
from  clay  or  wax. 
g,  outside. 


IO2 


THE   CELL 


new  cells  are  mainly  produced  by  division  from  pre- 
existing cells  at  and  near  the  tips  of  the  branches  of 
the  root  and  shoot.  These  regions  of  active  cell  division 
are  called  meristems*  The  cells  in  this  region  are  all 
young  because  they  have  all  recently  arisen  from  cell 
division.  The  characteristic  features  of  such  a  meris- 
tematic  or  embryonic  cell  (Fig.  6,  A)  are  that  the 
cell  wall  is  relatively  thin,  the  cell  cavity  is  completely 
filled  with  protoplasm  and  the  nucleus  is  large,  its 
diameter  commonly  being  as  much  as  two- thirds  to  three- 
quarters  that  of  the  whole  cell.  The  granular  endoplasm  2 


PIG.  7. — Enlarged  diagram  of  the  portion  X  of  Fig.  6.  C  :  c.w.,  cell  wall ; 
ect.,  ectoplasm;  endo,  endoplasm;  v.w.,  vacuole  wall;  pi.,  plastid. 
Supposed  to  be  multiplied  6,600  times. 

contains  various  granules  and  droplets  (metabolites) 
of  different  sizes.  It  also  contains  small  refringent 
bodies  called  plastids,  which  may  under  certain  cir- 
cumstances form  starch,  and  if  exposed  to  light  develop 
chlorophyll  and  become  chloroplasts.  The  endoplasm 
is  bounded  on  the  outside  by  the  very  thin  layer  of 
ectoplasm,  which  is,  however,  only  just  visible  under 
the  highest  powers  of  the  microscope  and  with  the 
best  optical  definition,  since  it  is  only  a  fraction  of 
i  p.  in  thickness  (Fig.  7). 
The  large  and  conspicuous  nucleus  (Fig.  6)  is  a 

1  Greek  (jLEpiarr^Q,  divider.  *  Svdov,  within. 


KARYOKINESIS  IO3 

spherical  body  bounded  by  the  nuclear  membrane  and 
containing  a  network  or  detached  granules  of  chromatin 
bathed  by  a  liquid,  the  nuclear  sap.  The  nuclear  mem- 
brane and  chromatin  are  doubtless  of  gel  structure,  the 
nuclear  sap  a  sol.  There  is  also  present  in  most  cases 
a  conspicuous  spherical  deeply  staining  body  (sometimes 
more  than  one)  called  the  nucleolus.  This  seems  to 
be  of  the  nature  of  reserve  material  which  is  used  to 
"  feed "  the  chromatin  when  division  occurs.  The 
nucleus  in  this  condition  is  often  called  the  "  resting 
nucleus,"  because  it  is  not  dividing,  but  the  [term*,  is 
rather  misleading,  for  the  nucleus  is  really  carrying  out 
its  main  function,  that  of  "  directing  "  the  nutrition 
and  growth  of  the  cell. 

Division  of  the  Nucleus  (Karyokinesis)  and  of  the 
Cell. — Meristematic  cells  constantly  divide,  and  between 
two  successive  divisions  there  is  a  period  of  growth 
(increase  in  bulk)  of  the  protoplasm  till  each  daughter 
cell  attains  about  the  size  of  the  mother  cell  before 
division,  when  each  divides  again.  The  division  of 
the  cell  is  always  preceded  by  division  of  the  nucleus, 
which  takes  place  in  a  remarkable,  complicated  fashion, 
the  process  being  known  as  karyokinesis  I  or  mitosis  2 
(Fig.  7,  G).  Except  in  the  nuclear  divisions  preceding 
the  formation  of  the  reproductive  cells,  this  process 
of  karyokinesis  shows  substantially  the  same  features 
in  every  nuclear  division  preceding  cell  division  in 
practically  all  animals  and  plants,  the  differences  met 
with  being  of  quite  minor  importance. 

The  chromatin  granules  (or  chromatin  network 
[Fig.  8,  a])  of  the  "  resting  nucleus  "  become  arranged  in 

1  Greek  Kapvov,  a  nut  (nucleus),  Klvrjaii;,  motion. 
1  From  Greek  (jnr6(a,  to  stretch  the  warp  in  the  loom,  because  of 
the  thread  structures  appearing  in  the  process. 


IO4  THE   CELL 

bent  chromatin  rods  of  equal  length,  called  chromosomes 
(Fig.  8,  b),  the  nucleolus  and  nuclear  membrane  l  dis- 
appearing. A  spindle-shaped  structure,  consisting  of 
exceedingly  delicate  protoplasmic  threads,  appears  in 
the  cell,  frequently  stretching  across  almost  its  entire 
width  (Fig.  8,  c).  This  spindle  is  called  the  achromatic 
spindle,  because  its  threads  do  not  take  up  stains  and 
become  deeply  coloured  as  do  the  chromosomes.  The 
threads  converge  on  two  spots  (poles).  In  some  plant 
cells  and  in  most  animal  cells  each  pole  is  occupied  by 
a  staining  granule  called  a  centrosome  to  which  the 
threads  are  attached.  The  chromosomes  now  become 
arranged  on  the  spindle  at  its  equator.  Each  chromo- 
some commonly  has  the  form  of  a  rod  bent  in  the  middle 
rather  like  a  hairpin,  the  free  ends  projecting  outwards, 
perpendicular  to  the  long  axis  of  the  spindle,  the  bend 
directed  towards  the  centre  of  the  spindle  (Fig.  8,  d). 
At  this  time  each  chromosome  is  seen  to  have  a  double 
structure,  the  bent  rod  being  split  longitudinally,  but 
this  splitting  may  occur  earlier,  so  that  the  double 
structure  is  visible  as  soon  as  the  chromosomes  appear 
from  the  resting  nucleus  (Fig.  8,  c).  Spindle  fibres 
running  from  the  poles  towards  the  equator  (and  distinct 
from  the  fibres  on  which  the  chromosomes  rest,  which 
run  continuously  from  pole  to  pole)  now  become 
attached  to  the  chromosomes  at  or  near  the  inwardly 
directed  bend  (Fig.  8,  A).  These  fibres  contract  and 
thus  pull  the  longitudinal  halves  of  the  chromosomes 
(daughter  chromosomes)  apart,  the  daughter  chromo- 
somes remaining  in  contact  longest  at  the  free  ends 
(Fig.  8,  e  and  B).  By  continued  contraction  of  the 

1  In  some  animal  cells  the  nuclear  membrane  remains  intact 
during  karyokinesis,  the  whole  of  the  processes  described  taking  place 
inside  it. 


KARYOKINESIS 


105 


achromatic    threads    the    daughter    chromosomes    are 
completely    separated   and    pulled    along  the   guiding 


FIG.  8. — Karyokinesis  in  a  meristematic  cell  (see  text). 
Modified  from  Strasburger. 

spindle  threads  (Fig.  8,  /)  to  the  neighbourhood  of  the 
poles,  where  the  chromosomes  lose  their  separateness  and 
individuality  (Fig.  8,  g),  each  group  becomes  surrounded 


106  THE   CELL 

by  a  nuclear  membrane,  and  two  daughter  nuclei  are 
thus  constituted  (Fig.  8,  h). 

At  the  same  time  (in  the  tissue  cells  of  plants), 
thickenings  of  the  achromatic  spindle  fibres  appear  in 
the  equatorial  plane  (Fig.  8,  g),  i.e.  midway  between 
the  two  daughter  nuclei,  and  grow  till  they  fuse  laterally, 
forming  a  plate  or  membrane  across  the  cell  (Fig.  8,  h), 
which  becomes  joined  to  the  lateral  cell  walls.  This 
membrane  then  becomes  converted  into  cell  wall  sub- 
stance, forming  a  new  cell  wall,  which  divides  the  original 
cell  into  two.  The  achromatic  spindle  then  disappears, 
all  except  the  equatorial  portion  on  which  the  thicken- 
ings which  initiated  the  new  cell  wall  were  formed. 
This  portion  of  the  threads,  penetrating  the  new  cell 
wall,  remain,  and  thus  maintain  continuity  between 
the  protoplasm  of  the  two  sister  cells.  This  continuity 
can  be  demonstrated  as  existing  between  the  protoplasm 
of  adult  living  cells. 

It  will  be  seen  that  the  process  of  karyokinesis  has 
the  effect  of  dividing  the  chromatin  of  the  nucleus 
accurately  into  halves  between  the  two  daughter  cells. 
It  is  believed  that  each  chromosome  carries  the  organic 
basis  of  certain  characteristics  of  the  cells  of  the  par- 
ticular species  of  organism,  and  that  when  each  is 
divided  longitudinally  into  halves,  these  characteristics 
are  equally  shared  by  the  two  halves  (daughter  chromo- 
somes), and  thus  each  daughter  cell  comes  to  have  the 
same  characteristics  as  the  mother  cell. 

Development  of  the  Adult  Cell  from  the  Embryonic 
(Meristematic)  Cell  (cf.  Fig.  6). — So  long  as  cells  remain 
meristematic  they  do  not  cease  to  pass  through  the 
rhythm  of  growth  and  division,  but  the  products 
of  cell  division  on  the  side  towards  the  body  of  the 
root  or  shoot  gradually  pass  out  of  the  meristematic 


VACUOLATION   AND    GROWTH   OF   CELLS  107 

state  and  assume  the  characters  of  permanent  tissue 
cells.  The  power  of  cell  division  does  not  necessarily 
cease,  but  divisions  become  less  frequent  and  in  some 
cases  do  not  recur. 

The  permanent  tissue  cells  commonly  attain  a  size 
far  greater  than  that  of  the  meristematic  cells — many 
of  them  grow  especially  in  the  direction  of  the  axis 
of  the  root  or  stem,  and  become  several  times  longer 
than  they  are  broad.  This  great  increase  in  the  size 
of  the  cell  does  not  result,  however,  from  increase  in 
bulk  of  the  protoplasm,  but  from  the  formation  and  increase 
in  size  of  vacuoLes.  Drops  of  liquid  (cell  sap)  appear 
in  the  endoplasm,  increase  in  size  (Fig.  6,  B),  and 
eventually  run  together,  forming  one  large  vacuole, 
in  the  midst  of  which  the  nucleus,  always  surrounded 
by  a  layer  of  cytoplasm,  is  suspended  by  strands  of 
cytoplasm  (bridles],  which  run  across  the  vacuole  to 
the  layer  lining  the  cell  wall.  When  the  vacuole 
increases  still  further  in  size  the  bridles  are  thinned 
out  till  they  collapse,  and  the  whole  of  the  cytoplasm 
then  forms  a  layer  on  the  cell  wall,  the  centre  of  the 
cell  being  completely  occupied  by  the  large  vacuole 
(Fig.  6,  C,  left-hand  bottom  cell). 

During  the  process  just  described  the  appearance 
and  increase  in  size  of  the  vacuoles  distends  the  ex- 
tensible cell  wall,  which  would  be  thinned  out  to  the 
point  of  rupture  were  it  not  that  new  cellulose  is 
continually  added  to  it  by  the  protoplasm  lining  its 
inner  surface.  When  the  cell  has  reached  its  definitive 
size  this  process  of  addition  of  cellulose  often  con- 
tinues, and  now  the  cell  wall  is  actually  thickened, 
the  successive  layers  added  to  its  inner  surface  being 
often  clearly  visible  on  each  side  of  the  original  thin 
wall  (middle  lamella}.  This  thickening  is  not,  however, 


IO8  THE   CELL 

always  quite  continuous  over  the  whole  surface.  At 
certain  spots  the  wall  may  not  be  thickened,  so  that 
channels  or  pits  are  leit,  leading  from  the  middle  lamella 
to  the  cavity  of  the  cell.  The  pits  of  adjacent  cells  are 
practically  always  formed  opposite  to  one  another.  This 
arrangement  greatly  facilitates  the  passage  of  water 
and  solutes  from  cell  to  cell. 

Osmotic  Pressure  and  Turgor. — What  is  the  force 
which  causes  the  distension  of  the  cell  during  its  growth 
from  the  embryonic  to  the  adult  condition  ?  It  is  the 
pressure  (often  called  "  osmotic  pressure  ")  caused  by 
the  "  attraction  "  of  osmotic  substances  within  the  cell 
for  water,  which  is  drawn  into  the  cell  in  the  effort 
to  establish  equilibrium  (cf.  p.  57),  and  develops  a  liquid 
pressure  against  the  wall.  We  must  suppose  that  the 
vacuoles  are  initiated  in  the  protoplasm  by  the  local 
concentration  of  an  osmotic  substance,  e.g.  sugar,  in 
certain  spots,  perhaps  within  plastids.1  Water  is 
attracted  to  these  spots  and  more  water  enters  the  cell 
from  without  to  replace  what  is  thus  removed  from  the 
general  cytoplasm.  As  the  pressure  in  the  vacuoles 
increases  the  cytoplasm  is  pressed  against  the  cell 
wall,  which  is  itself  progressively  distended,  just  as  the 
inner  tube  of  an  inflated  bicycle  tyre  is  pressed  against 
the  outer  cover.  If  the  cell  were  freely  suspended  in 
water  this  process  would  continue  till  the  wall  burst 
or  till  the  elastic  reaction  of  the  wall  counterbalanced 
the  distending  force  due  to  the  attraction  of  the  osmotic 
substance  for  water.  But  when  the  cell  is  surrounded 
by  other  cells  similarly  distended  they  all  press  against 
one  another  and  give  rigidity  to  the  whole  tissue. 
This  condition  of  a  tissue  is  called  turgor  or  turgidity, 

1  In  any  case  a  semi-permeable  membrane  is  formed  round  each 
vacuole,  whether  derived  from  a  plastid  or  formed  in  the  endoplasm. 


WILTING   AND    PLASMOLYSIS  lOQ 

and  herbaceous  plants  depend  upon  it  for  maintain- 
ing their  stiffness  and  the  erect  position  of  their 
shoots. 

Wilting  and  Plasmolysis. — If  a  plant  cell  loses  water 
faster  than  it  can  obtain  it,  turgidity  cannot  be  main- 
tamed,  the  cell  becomes  soft  and  limp,  and  the  tissue 
loses  its  rigidity.  This  is  what  happens  on  a  very 
hot  day  when  plants  lose  water  by  evaporation  to  the 
air  faster  than  they  can  replenish  their  supply  by  root 
absorption.  The  plant  is  said  to  wilt.  The  loss  of 
turgidity  takes  place  even  more  rapidly  when  a  plant 
is  pulled  up  by  the  roots  and  left  in  the  sun. 

Turgidity  can  also  be  lost  in  another  way.  If  the 
cell  or  tissue  be  placed  in  a  sugar  or  salt  solution  of 
osmotic  strength  greater  than  that  of  the  cell  sap, 
water  is  drawn  out  of  the  cell  against  the  pull  of  the 
cell  sap.  The  protoplasm  is  then  no  longer  held  tight 
against  the  wall  by  the  pressure  from  within,  and  it 
contracts  away  from  the  wall  as  the  liquid  pressure 
within  diminishes  by  the  withdrawal  of  water.  This 
process  is  called  plasmolysis.1  The  osmotic  substances 
of  the  cell  sap  do  not,  however,  pass  out  through  the 
semi-permeable  membranes  represented  by  the  vacuole 
wall  and  ectoplasm,  and  if  the  cell  or  tissue  be  placed 
again  in  water  this  re-enters  the  cell,  whose  turgidity 
is  re-established. 

PRACTICAL  WORK. 

(i)  Examine  the  growth  of  the  bean  roots  set  up  in  the  corked 
bottle  last  time.  They  show  (a)  local  growth,  (b)  curvature 
from  the  horizontal  to  the  vertical  plane,  i.e.  the  bending  of  an 
organ  in  response  to  an  external  force.  Sketch  diagrammatically 
these  results  alongside  the  sketches  of  the  roots  as  originally 
set  up. 

»  Greek  Xvaiq,  loosening  of  the  plasma  from  the  wall. 


110  THE   CELL 


THE  PLANT  CELL. 

(2)  Strip  off  the  surface  layer  of  cells  from  the  piece  of  onion 
scale  provided.  Examine  and  draw  carefully  under  the  high 
power  two  or  three  of  the  cells,  marking  the  parts. 

(3) x  Tease  out  in  a  drop  of  water  a  little  of  the  pulp  of  the 
Snowberry,  taking  care  that  it  is  kept  well  wetted.  Note  that 
the  tissue  is  loose,  with  many  air  spaces  between  the  cells,  many 
of  which  are  spherical.  Examine  the  cells  under  the  high  power. 
Note  that  the  proportion  of  cell  sap  to  protoplasm  in  each  cell  is 
very  high.  Look  for  the  nucleus.  Staining  with  dilute  iodine 
solution  will  often  help  in  case  of  difficulty. 

(4)  Examine   the  stained  longitudinal  section  of  the  tip  of  a 
bean  root.      Note  that  the  actual  tip  is  formed  by  the  "  root-cap." 
The  area  just  behind  this  is  the  "  growing  point "  or  primary 
meristem,  and  consists  of  small  mostly  square  cells  densely  filled 
with  protoplasm  and  with  large  conspicuous  densely  staining 
nuclei.     Draw  carefully  under  the  high  power  (a)  examples  of 
meristematic  cells  with  "  resting  nuclei,"  (b)  different  stages    of 
nuclear    division  *    (compare    with    demonstration    slides),    (c) 
different  stages  of  vacuolation  of  the  cells  behind  the  meristem. 

Measure  the  diameter  of  a  meristematic  cell  and  of  its  nucleus, 
also  the  length  of  one  of  the  longest  cells  in  the  section. 

CELLS  OF  ANIMAL  TISSUED 

(5)  Examine   the   demonstration   specimens    of    stained   and 
mounted  animal  tissues.3     Note  cytoplasm,  nucleus  and  absence 
of  cell  wall :    also,  e.g.  in  cartilage,  the  intercellular  substance 
or  "  matrix." 

TURGIDITY   AND    PLASMOLYSIS. 

(6)  Place  one  of  the  turgid  bean  roots  (a)  in  10  per  cent.  CaCl2 
solution, 4   leaving   the   other   (b)    on   the   table.     After   a    few 
minutes  note  that  (a)  has  become  quite  limp.     Place  it  in  water, 
and  it  gradually  regains  its  turgidity;  (b)  gradually  wilts  as  it 
loses  water  by  evaporation  to  the  air. 

1  This  may  be  omitted  if  the  Practical  Work  is  too  long. 
1  Demonstration  slides  under  immersion  objectives  showing  stages 
in  karyokinesis  should  be  available  if  possible. 

3  Three  or  four  slides  will  suffice  :    columnar   epithelium,  cartilage, 
connective  tissue  and  a  preparation  of  muscle  showing  nuclei  are 
suggested  as  suitable. 

4  NaCl  solution  is  liable  to  injure  the  protoplasmic  membranes. 


PRACTICAL   WORK  III 

(7)  With  a  knife  split  a  length  of  fresh  bean  stem  longitudinally 
into  quarters.     Note  that  the  ends  of  the  strips  curve  outwards. 
The  harder  tissues  of  the  outside  of  the  stem  are  in  a  state  of 
elastic  tension  in  the  intact  stem,  i.e.  they  are  tending  to  contract 
in  the  longitudinal  direction,  but  cannot  do  so  against  the  swelling 
force  of  the  turgid  cells  of  the  centre  (pith)  of  the  stem,  which 
are    similarly    prevented    from    actually    swelling.     When    the 
stem  is  divided  longitudinally  the  inner  cells  swell  and  the  outer 
ones  contract,  so  that  each  strip  of  stem  bends  outwards. 

(8)  Strip  off  a  fragment  of  the  coloured  surface   layer  of 
Tradescantia  leaf  as  free  as  possible  from  the  green  tissue  below. 
Mount  in  water  and  examine  under  the  high  power.     The  vacuoles 
of  the  purple  cells  are  filled  with  coloured  cell  sap.     Place  a  drop 
of  10  per  cent.  CaCl£  solution  on  the  slide  just  touching  the  edge 
of  the  coverslip,  and  draw  the  solution  under  the  coverslip  by 
holding  a  fragment  of  blotting  paper  against  the  opposite  edge. 
Watch  the  effect  on  the  cells — plasmolysis.     Reverse  the  action 
by  running  in  a  drop  of  water  in  the  same  way  and  watch  the 
recovery  of  turgor. 


CHAPTER   VII 

THE   GREEN   PLANT  CELL 

Chloroplasts  and  Chlorophyll.  —  The  majority  of 
plants  we  are  familiar  with  have  green  stems  and 
leaves,  or  green  leaves  alone.  The  green  colour  is 
due  to  the  mixed  pigment  chlorophyll  present  in  the 
chloroplasts  (which  are  definite,  oval,  spherical  or  disc 
shaped,  protoplasmic  bodies  lying  in  the  cytoplasm  of 
certain  of  the  living  cells  of  the  leaf  or  stem),  and  in 
these  alone.  The  lower  green  plants  (for  instance  the 
Mosses  and  Liverworts)  possess  chloroplasts  of  the 
same  nature  as  those  of  the  seed  plants,  but  in  the 
Green  Algae  the  chloroplasts  are  very  various  in  shape 
and  size  (e.g.  spiral  bands,  Fig.  9).  The  chloroplasts  are 
derived  from  the  plastids  (which  multiply  by  simple 
division)  found  in  the  meristematic  cells,  and  are  not 
formed  afresh  in  the  protoplasm. 

Chlorophyll  is  a  mixture  of  four  different  pigments 
held  in  the  protoplasm  of  the  chloroplast  in  a  colloid 
state :  chlorophyll  a  (C55H72O5N4Mg)  giving  a  blue- 
green  solution,  chlorophyll  ft  (C55H7006N4Mg)  giving 
a  pure  green  solution,  carotin  l  (C40H56)  giving  orange 
crystals,  and  xanthophyll  (C40H56O2)  giving  yellow 
crystals.  These  are  mixed  in  the  chloroplasts  in  the 
proportion  of  six  molecules  of  the  two  green  pigments 
to  two  of  the  two  orange  and  yellow  ones. 

1  Carotin  also  occurs  in  other  parts  of  various  plants.  It  is,  for 
instance,  the  red  colouring  matter  of  carrots,  of  many  red  and  orange 
coloured  flowers,  etc. ;  also  of  the  "  eyespot "  in  Chlamydomonas 
(see  p.  1 86). 


SPECTRUM    OF   CHLOROPHYLL  113 

Photosynthesis. — The  spectrum  of  chlorophyll  shows 
definite  absorption  bands.1  These  absorption  bands 
represent  the  rays  most  strongly  absorbed  by  the 
chlorophyll,  and  thus  the  radiant  energy  available 
for  the  work  which  the  chloroplast  carries  on.  That 
these  are  the  rays  actually  used  in  photosynthesis, 
or  at  least  in  one  or  more  of  the  processes  that  go  to 
make  up  photosynthesis,  can  be  shown  by  a  very  pretty 
experiment  first  devised  by  the  German  botanist 
Engelmann.  A  thread  of  green  alga  living  and  carry- 
ing on  its  work  in  water  is  illuminated  by  a  solar  spec- 
trum so  that  different  parts  of  the  thread  are  illuminated 
by  different  coloured  rays.  A  culture  of  a  certain 

pte3[ Orange      Yellow    Green      BW  VioUt 

a\ 


FIG.  9. — Diagram  illustrating  Engelmann's  experiment,  to  show  by 
the  aggregation  of  a  motile  bacterium  (b)  highly  sensitive  to  oxygen 
the  regions  of  maximum  evolution  of  oxygen  by  a  green  alga  (a— a) 
illuminated  by  a  solar  spectrum.  The  most  active  rays  correspond 
with  the  red  and  orange. 


bacterium    (Bacterium  termo)    which   is    very    sensitive 
to  oxygen,  so  that  its  cells  move  towards  any  source 

1  When  a  beam  of  white  light  is  split  up  by  passing  through  a  prism 
into  a  band  or  spectrum  of  differently  coloured  rays,  owing  to  the 
different  angles  through  which  the  different  rays  are  bent  by  the 
prism,  and  is  allowed  to  fall  on  a  white  surface,  the  spectrum  so  formed 
(solar  spectrum)  is  continuous.  But  if  the  beam  has  passed  through 
a  translucent  coloured  substance,  the  spectrum  formed  shows  dark 
bands  (absorption  bands)  corresponding  inversely  with  the  colour  of 
the  translucent  substance,  the  colour  of  the  substance  being  caused 
by  the  combined  effect  on  the  eye  of  the  coloured  rays  that  have 
passed  through  it.  The  absorption  bands  are  caused  by  the  absence 
of  the  rays  that  have  been  stopped  or  absorbed  by  the  coloured 
substance.  Thus  a  pure  red  substance  shows  absorption  in  the  other 
colours  of  the  spectrum — blue,  green,  etc.,  a  blue  substance  in  the 
red,  orange,  etc.,  and  so  on. 

8 


114  THE  GREEN  PLANT  CELL 

of  oxygen  diffusing  into  the  water,  is  then  introduced 
in  the  neighbourhood  of  the  illuminated  algal  thread. 
The  minute  cells  of  the  bacteria  swarm  towards  and 
cluster  round  those  portions  of  the  thread  illuminated 
by  the  coloured  rays  corresponding  with  the  chief 
absorption  band  of  chlorophyll,  especially  in  the  red 
(Fig.  9). 

This  behaviour  of  the  bacteria  shows  that  free  oxygen 
(one  of  the  products  of  the  process  of  photosynthesis) 
is  being  given  off  most  actively  at  those  points ;  and 
consequently  that  those  particular  rays  are  the  ones 
most  active  in  photosynthesis. 

Sugar,  probably  glucose,  is  the  first  product  of  the 
process  of  photosynthesis  that  can  be  definitely  recog- 
nised in  the  green  cells  of  the  leaf,  free  oxygen  being 
liberated  at  the  same  time.  These  substances  are  only 
formed  in  the  presence  of  a  certain  intensity  of  the  proper 
rays  of  the  spectrum  (pure  white  sunlight  contains  all 
the  rays),  and  in  the  presence  of  carbon  dioxide  and 
water.  We  may  represent  the  general  equation  of 
photosynthesis  thus  : — 

6C02  +  6H20  m  C6H1206  +  602 

It  is  likely,  however,  that  formaldehyde  is  first  formed  : — 
CO2  +  H2O  =  CH2O  +  O2 

and  that  the  formaldehyde  is  then  condensed  to  form 
glucose  : — 

6CH20  =  C6H1206 

but  this  is  uncertain,  since  formaldehyde  has  not  been 
demonstrated  in  the  leaf.  Nevertheless  the  "  reduc- 
tion "  of  CO2  in  presence  of  water  to  form  formaldehyde 
and  also  the  condensation  of  formaldehyde  to  form 
glucose  are  both  operations  that  can  be  performed  in 


STRUCTURE  OF  MESOPHYLL  115 

the  chemical  laboratory.  For  the  first  process  consider- 
able energy  is  required,  and  it  is  this  that  certainly 
requires,  in  the  chloroplast,  the  radiant  energy  of 
light.  This  first  process  may  then  be  designated  as 
photolysis,  the  second  process,  that  of  condensation  to 
form  sugar  being  a  process  of  chemo synthesis.  For 
ordinary  biological  purposes,  however,  we  may  consider 
them  together  as  photosynthesis. 

Structure  and  Functions  of  the  Green  Plant  Cell.— 
The  green  tissue  (mesophyll  l)  which  forms  the  bulk  of 
an  ordinary  foliage  leaf  (see  Fig.  10)  consists  of  cells 
with  thin  cellulose  walls  lined  inside  by  a  layer  of 
cytoplasm  enclosing  the  nucleus  and  numerous  chloro- 
plasts,  and  with  a  large  central  vacuole.  These  cells 
are,  in  fact,  "  adult  "  living  cells  (see  p.  101)  containing 
chloroplasts  (Fig.  n). 

The  mesophyll  tissue  is  covered  by  a  layer  of  colour- 
less living  cells  (epidermis)  on  the  surface  of  the  leaf, 
and  the  outer  walls  of  these  cells,  in  contact  with  the 
external  air,  has  a  waterproof  layer  of  cell  wall  sub- 
stance (cuticle)  which  prevents  the  cells  of  the  leaf 
drying  up  by  evaporation  to  the  air  (Fig.  10,  st.)  The 
epidermis  (and  cuticle)  is,  however,  pierced  by  a  number 
of  holes  (stomata 2)  which  lead  from  the  external  air 
to  the  air-containing  intercellular  spaces  between  the 
mesophyll  cells.  These  intercellular  spaces  are  inter- 
communicating, and  are  so  arranged  that  every  meso- 
phyll cell  has  some  part  of  the  outer  surface  of  its  wall 
in  contact  with  the  internal  atmosphere  of  the  leaf. 

The  internal  atmosphere  of  the  leaf,  being  in  com- 
munication with  the  external  air  through  the  stomata, 
has  at  first  the  same  composition  as  the  latter, 
but  this  composition  is  continually  being  modified  by 

1  Greek  jue'aoj,  middle,  and  ^vAAov,  a  leaf.          *  oro/ia,  mouth. 


Il6  THE   GREEN   PLANT  CELL 

the  interchange  of  gases  between  it  and  the  mesophyll 
cells,  and  equilibrium  between  it  and  the  external 
air  is  constantly  tending  to  be  re-established  by  the 
diffusion  of  gases  through  the  stomata,  into  or  out  of 
the  leaf.  Thus  if  the  mesophyll  cells  absorb  carbon 
dioxide,  as  they  do  when  illuminated,  the  pressure  of 
this  gas  decreases  in  the  intercellular  spaces  and  fresh 
carbon  dioxide  from  the  air  streams  in  through  the 


FIG.  10. — Diagram  of  transverse  section  of  a  leaf  to  show  the  relation 
of  the  mesophyll  (mes.)  to  the  system  of  intercellular  spaces  (int.) 
and  the  stomata  (st.).  The  white  arrows  represent  the  evapora- 
tion of  water  from  the  mesophyll  cells  into  the  intercellular 
spaces,  and  its  diffusion  through  the  stomata  to  the  outer  air. 


stomata  to  restore  the  equilibrium.  The  oxygen  pro- 
duced in  the  mesophyll  cells  under  the  same  conditions 
increases  the  pressure  of  oxygen  in  the  intercellular 
spaces,  and  it  escapes  through  the  stomata  into  the 
external  air.  The  air  in  the  intercellular  spaces  is 
normally  nearly  saturated  with  water  vapour,  owing 
to  the  constant  evaporation  of  water  from  the  wet 
mesophyll  cells  into  the  air  in  contact  with  them, 
and  this  water  vapour  is  always  escaping  through  the 


FUNCTION   OF   CHLOROPLAST 


117 


stomata  if  the  outer  air  is  drier,  as  it  usually  is,  than 
the  air  inside  the  leaf  (Fig.  10,  white  arrows). 

Let  us  consider  now  what  happens  in  the  chloroplast 
of  a  mesophyll  cell  which  is  suddenly  well  illuminated. 
The  light  falling  upon  it  is  robbed  of  those  rays  of  the 
spectrum  corresponding  with  the  absorption  bands 


-Two  mesophyll  (palisade)  cells. 
Dne  on  the  left  is  seen  in  optical 


FIG.  ii.— : 

The 

section,  the  one  on  the  right  in  sur- 
face view.  Above  are  two  cells  of 
the  upper  epidermis,  chl.,  chloro- 
plast; n,  nucleus;  c.w.,  cell  wall; 
ep.,  epidermal  cell ;  int.,  intercellular 
space. 


FIG.  12. — A,  diagram  of  chlo- 
roplast of  mesophyll  cell 
to  show  its  activity  when 
illuminated  ;  v,  vacuole  ; 
int..  intercellular  space 
B,  starch  grains  formed 
in  chloroplast. 


of  chlorophyll.  The  energy  so  obtained  is  used  to 
split  up  the  molecules  of  carbon  dioxide  provided  by 
the  respiration  of  the  protoplasm,  and  the  carbon  is 
united  with  th©  elements  of  water  (always  present  in 
the  cell)  with  the  ultimate  formation  of  sugar,  oxygen 
at  the  same  time  being  liberated  (Fig.  12,  A).  Some 
of  the  sugar  and  oxygen  so  formed  is  doubtless  used 


Il8  THE  GREEN  PLANT  CELL 

in  the  respiration  of  the  colourless  protoplasm,  but, 
under  favourable  conditions,  the  rate  of  photosynthesis 
far  exceeds  the  rate  of  respiration  in  a  green  cell.  This 
means  in  the  first  place  that  far  more  carbon  dioxide 
is  used  than  can  be  obtained  from  the  process  of  respira- 
tion, the  pressure  of  the  gas  in  the  cell  decreases  and 
fresh  supplies  enter  from  the  air  of  the  intercellular 
spaces.  This  in  turn  causes  an  inflow  of  carbon  dioxide 
through  the  stomata. 

The  oxygen  formed  as  a  by-product  of  photosynthesis 
increases  the  pressure  of  this  gas  in  the  cell,  and  is  given 
off  at  the  cell  surface  into  the  intercellular  spaces  and 
thence  diffuses  through  the  stomata  into  the  open  air, 
so  that  well  illuminated  leaves  are  constantly  giving 
off  free  oxygen.  The  sugar  continuously  produced 
by  the  chloroplasts  diffuses  into  the  cell  sap.  Some  of 
it  passes  into  the  bundles  (veins)  of  the  leaf  and  along 
the  bundles  to  other  parts  of  the  plant.  But  more 
sugar  is  produced  than  can  be  removed  from  the  meso- 
phyll  by  this  means,  and  the  concentration  of  sugar  in 
the  cell  continuously  rises  during  the  day. 

Formation  of  Starch  in  the  Chloroplasts.— In  most 
green  plants,  though  by  no  means  in  all,  the  excess  of 
sugar  after  a  certain  degree  of  concentration  is  reached, 
is  converted  into  starch  by  condensation,  and  this 
forms  small  grains  in  the  substance  of  the  chloroplast : — 

«C6H1206  =  (C6H1005)rt  +  «H20 

A  foliage  leaf  of  a  starch-forming  plant  at  the  close 
of  a  bright  day  when  large  amounts  of  sugar  have 
been  formed  shows  its  chloroplasts  packed  with  starch 
grains  (Fig.  12,  B),  as  can  easily  be  seen  by  examining 
a  section  of  the  leaf  under  the  high  power.  The  starch 
grains  appear  as  bright  granules  in  the  substance  of 


STARCH    FORMATION  IIQ 

the  chloroplast.  The  mass  of  starch  formed  in  the 
leaf,  and  the  dependence  of  this  process  upon  illumina- 
tion, can  be  strikingly  demonstrated  to  the  naked  eye 
by  covering  part  of  the  leaf  during  the  day  with  tinfoil 
while  still  on  the  plant  and  leaving  the  rest  exposed. 
If  in  the  afternoon  the  leaf  is  cut  off,  killed  with  boiling 
water,  the  colour  taken  out  with  alcohol,  and  the  leaf 
is  then  placed  in  a  dish  and  flooded  with  iodine  solution, 
the  part  of  the  leaf  that  was  exposed  to  light  will 
quickly  turn  blue-black  owing  to  the  staining  of  the 
starch  grains  as  the  iodine  gradually  penetrates  the 
tissues,  while  that  part  of  the  leaf  protected  from 
light  remains  colourless. 

The  function  of  the  chloroplasts  as  starch  formers 
is  quite  distinct  from  their  function  as  sugar  producers. 
It  does  not  depend  directly  on  the  photosynthesis  of 
sugars  or  on  the  green  colour  of  the  chloroplasts,  but 
on  the  actual  concentration  of  sugar  in  the  leaf.  The 
leaves  of  many  plants  do  not  form  starch  at  all,  and 
the  concentration  of  sugar  in  the  leaf  necessary  to  lead 
to  starch  formation  varies  considerably  in  different 
species.  The  power  of  forming  starch  from  sugar  is 
shared  by  the  colourless  plastids  (leucoplasts)  found 
in  the  colourless  cells  of  the  plant,  particularly  in  seeds, 
tubers,  etc.,  but  also  in  stems  and  roots,  where  large 
quantities  of  starch  are  formed  from  the  sugar  which 
comes  to  these  cells  from  the  leaves.  Chloroplasts 
themselves  can  form  starch  from  sugar  which  they 
have  not  made,  as  can  be  proved  by  the  experiment 
of  floating  detached  living  leaves  on  sugar  solution  in 
the  dark,  when  starch  will  appear  in  the  chloroplasts. 

As  the  illumination  decreases  the  rate  of  sugar 
formation  likewise  decreases,  and  when  it  falls  below  a 
certain  intensity — in  natural  illumination  about  sunset — 


120  THE  GREEN  PLANT  CELL 

the  photosynthesis  of  sugar  becomes  negligibly  smalls 
Since  a  certain  amount  of  sugar  is  always  being 
removed  from  the  cell  through  the  veins,  the  concentra- 
tion of  sugar  in  the  mesophyll  cells  falls,  and  the  starch 
grains  in  the  chloroplasts  are  gradually  reconverted 
into  sugar  during  the  night,  and  this  sugar  is  likewise 
conveyed  away  by  the  veins.  Thus  both  during  the 
day  and  during  the  night  there  is  a  constant  stream  of 
sugar  away  from  the  leaf  to  other  parts  of  the  plant 
where  it  is  used  in  respiration,  in  the  formation  of  new 
protoplasm  and  new  cell  walls,  or  is  stored  as  starch,  or 
sometimes  remains  as  sugar  (beetroot,  sweet  fruits, 
etc.),  or  as  some  other  carbohydrate  such  as  inulin 
(Jerusalem  artichoke,  etc.),  or  is  converted  into  fats. 

The  formation  of  glucose  from  starch  and  of  starch 
from  glucose  may  be  represented  by  the  general 
reversible  equation  :  — 

condensation 

«C6H1206  ^±   (C6H1005)W  +  «H20 

hydrolysis 

The  reaction  is  catalysed  by  the  enzymes  diastase 
and  maltase,  probably  in  either  direction  (see  p.  45 
for  the  intermediate  substances  formed). 

Photosynthesis  in  a  Green  Cell  living  in  Water.— In 
a  green  cell  living  in  water  the  processes  of  sugar  and 
starch  formation  are  essentially  the  same  as  in  the 
mesophyll  cell  of  a  leaf.  The  only  difference  is  that 
the  carbon  dioxide  is  derived  directly  from  that  which 
is  dissolved  in  the  water  instead  of  being  absorbed 
from  the  air,  and  that  the  sugar  does  not  leave  the  cell. 
Starch  (or  in  some  species  fat)  is  formed  when  the 
concentration  of  sugar  passes  a  certain  point.  The  sugar 
is  used  for  respiration,  to  form  the  cellulose  for  cell 
wall  substance,  and  also  (together  with  the  elements  of 


RESPIRATION   AND   PHOTOSYNTHESIS  121 

mineral  salts  entering  the  cell  from  the  surrounding  water), 
to  make  the  proteins  which  form  the  basis  of  the  new 
protoplasm  produced  as  the  cell  grows  and  divides. 

Respiration  and  Photosynthesis. — It  will  be  noted 
that  the  general  equation  representing  respiration  is 
the  exact  reverse  of  that  representing  photosynthesis. 
The  two  equations  may  be  combined  thus  : — 

RESPIRATION 
potential  energy  — >  liberation  of  energy  — >  kinetic  energy 

(oxidation) 
C6H1206  +  602    ^=±     6C02  +  6H20 

(reduction) 

potential  energy  < —  locking  up  of  energy  < —  kinetic  energy 
PHOTOSYNTHESIS 

In  the  mesophyll  cell  of  a  well-lighted  leaf  the  photo- 
synthetic  process  very  greatly  exceeds  respiration, 
so  that  carbon  dioxide  and  water  are  continuously 
used  up,  sugar  and  free  oxygen  produced.  When 
illumination  falls  to  a  certain  low  level  of  intensity 
the  two  processes  will  exactly  balance,  and  the  system 
represented  by  the  green  cell  will  be  in  equilibrium  in 
respect  of  these  processes.  In  still  weaker  light  or 
in  the  dark  photosynthesis  stops  altogether  and  sugar 
and  oxygen  are  continuouly  used  up,  carbon  dioxide 
and  water  produced.  Thus  it  is  true  that  green  plants 
give  off  carbon  dioxide  in  the  dark.  But  the  popular 
notion  that  it  is  "  unhealthy  "  to  keep  plants  in  a  bed- 
room at  night  is  nevertheless  unfounded — the  amount 
of  the  gas  given  off  as  a  result  of  the  respiration  of 
the  plant  cells  is  far  too  small  to  raise  appreciably 
the  carbon  dioxide  content  of  the  air. 

Synthesis  of  Proteins. — The  formation  of  sugars  in 
the  green  cells,  their  condensation  into  starch,  and  the 


122  THE  GREEN  PLANT  CELL 

hydrolysis  of  this  to  sugars  again,  form  a  very  important 
part  of  the  metabolism  of  green  plants.  Sugar  is  not 
only  used  by  the  plant  in  respiration,  but  it  is  also  the 
material  from  which  the  cell  walls,  the  skeleton  of  the 
plant,  are  formed.  For  these  reasons  the  carbohydrate 
metabolism  of  the  plant  makes  up  much  the  greater 
part  of  the  whole  of  its  metabolic  processes.  But  the 
synthesis  of  proteins  to  form  the  basis  of  the  new 
protoplasm  which  is  constantly  being  formed  in  meri- 
stematic  cells  is  clearly  also  of  essential  importance. 
This  appears  to  take  place  by  the  interaction  of  soluble 
carbohydrates  with  the  nitrogen  and  sulphur  derived 
from  the  nitrates  and  sulphates  absorbed  by  the  roots. 
It  seems  that  comparatively  simple  organic  nitrogenous 
substances  called  ammo-acids  are  thus  formed,  and 
that  these  are  synthesised  by  successive  condensations 
(analogous  to  the  condensations  of  sugar  to  starch  or 
cellulose)  into  more  complex  nitrogenous  substances, 
and  eventually  into  proteins.  These  anabolic  processes 
probably  occur  in  the  mesophyll  cells  of  the  leaf,  whence 
the  nitrogenous  substances  are  conveyed,  in  relatively 
simple  form,  to  growing  organs  and  storage  organs. 
At  least  the  final  synthesis  of  the  protoplasmic  proteins 
must  also  take  place  in  the  meristematic  cells  where 
new  protoplasm  is  actually  being  formed.  It  is  not 
possible  to  enter  here  into  the  details  of  these  processes, 
some  of  which  are  still  largely  obscure  and  others 
unknown.  Some  of  the  simpler  proteins — the  poly- 
peptides — can  be  made  synthetically  in  the  laboratory. 
The  Raw  Materials  of  Plant  Food. — The  chemical 
elements  which  are  constituents  of  the  molecules  of  the 
most  important  organic  substances  involved  in  the 
structure  of  organisms  have  been  enumerated  on  page  37. 
Of  these  the  green  plant  obtains  carbon  from  the  carbon 


RAW    MATERIALS   OF   FOOD  123 

dioxide  of  the  air,  hydrogen  and  oxygen  largely  from 
water,  nitrogen,  sulphur  and  phosphorus  from  nitrates, 
sulphates  and  phosphates  dissolved  in  the  water  of  the 
soil.  The  remaining  elements  essential  to  the  nutrition 
of  plants  are  potassium,  magnesium,  calcium  and  iron, 
and  these  are  obtained  from  the  same  mineral  salts. 
Other  elements,  such  for  instance  as  sodium,  silicon, 
etc.,  are  habitually  absorbed  by  the  plant,  but  they  are 
not  essential  as  materials  of  food.  This  can  be  proved 
by  growing  the  plant  not  in  soil  but  in  a  mixed  solution 
of  various  salts.  Such  water  cultures,  as  they  are  called, 
show  that  a  plant  can  grow  and  flourish  when  it  is  pro- 
vided with  a  selection  of  soluble  salts  in  dilute  watery 
solution  containing  only  the  elements  named.  A  good 
"  complete  "  water  culture  solution  is  the  following  : — 

Calcium  nitrate          . .  . .  . .     4  grams 

Potassium  nitrate      . .  . .  i  gram 

Magnesium  sulphate  . .  i  gram 

Potassium  phosphate  . .  i  gram 

Distilled  water  . .  . .  . .   50  c.c. 

This  is  a  stock  solution.  2  or  3  c.c.  should  be  used 
for  i  litre  of  culture  solution,  distilled  water  being 
added  to  make  up  the  litre,  and  a  drop  of  iron  chloride 
added.  Other  soluble  salts  can  be  substituted  provided 
they  contain  the  same  elements,  for  instance  potassium 
nitrate,  calcium  phosphate,  calcium  sulphate,  magnesium 
chloride. 

But  if  one  of  the  elements  represented  in  the  salts 
is  omitted  from  the  solution  the  plant  does  not  flourish, 
and  eventually  dies.  The  most  rapid  failure  results 
from  the  omission  of  nitrogen,  and  this  is  clearly  because 
nitrogen  is  an  essential  constituent  of  the  protein 
molecules.  When  the  store  of  combined  nitrogen  in 
the  seed  has  been  used  up  by  the  growing  plant,  no 


124 


THE  GREEN   PLANT  CELL 


43125 

FIG.  13. — Water  cultures  of  Buckwheat  (after  Nobbe).  i,  plant 
grown  in  complete  solution ;  2,  without  potassium ;  3,  with 
sodium  instead  of  potassium ;  4,  without  calcium ;  5,  without 
nitrogen. 


PRACTICAL   WORK  125 

new  protoplasm  can  be  formed,  and  growth  necessarily 
ceases  unless  fresh  supplies  of  combined  nitrogen  in 
a  suitable  form,  i.e.  as  nitrate,  can  be  obtained.  The 
ordinary  green  plant  cannot  directly  use  the  free  gaseous 
nitrogen  of  the  air  to  build  up  its  proteins.  And  the 
plant  cannot  develop  successfully,  and  dies  sooner  or 
later,  in  the  absence  of  any  one  of  the  other  essential 
elements. 

PRACTICAL  WORK. 

(1)  Soak  the  leaf  provided  in  alcohol  and  note  that  a  green 
pigment  is  gradually  extracted  from  the  leaf.     After  an  hour's 
soaking  pour  some  of   the  green  solution  into  a  test  tube  and 
examine  with  the  hand  spectroscope.     Compare  with  (6) . 

(2)  Strip  off  part  of  the  colourless  surface  layer  of  cells  (epi- 
dermis) of  the  leaf  of  Gladiolus  provided.     Below  you  will  see 
colourless  ribs    (veins)   with   strips  of   green  tissue  (mesophyll) 
between.     Cut  out  a  small  portion  of  this  green  tissue  with  the 
point  of  a  sharp  scalpel  or  penknife  and  mount  in  a  drop  of 
water,  taking  care  to  wet  the  tissue  thoroughly.     Put  on  a  cover- 
slip  and  examine  under  the  low  power.     Note  the  small  cells  with 
green  dots,  and  the  air  spaces  (now  partly  filled  with  water)  between 
them.     Examine  with  the  high  power  and  carefully  draw  one 
of  the  cells  of  which  you  can  get  a  clear  view,  marking  the  cell 
wall,  the  cytoplasm  lining  the  wall  with  chloroplasts  embedded  in 
it,  the   central  vacuole,  and  the  small  shining  spherical  nucleus. 
Stain  a  portion  of  the  green  tissue  with  haematoxylin  for  ten 
minutes  and  note  that  the  nucleus  especially  absorbs  and  becomes 
coloured  by  the  stain. 

(3)  Examine  under  the  microscope  in  a  drop  of  water  a  whole 
leaf  of  the  water  weed  Elodea.      This  is  a  "  water  leaf,"  wholly 
submerged  in  nature  and  of  very  simple  structure,  consisting, 
except  for  the  veins,  of  only  two  layers  of  cells,  all  containing 
chloroplasts.     The  structure  of  each  cell  is  essentially  the  same 
as  that  of  the  mesophyll  cell  of  Gladiolus,  though  the  cells  are 
much  larger  and  oblong  in  shape.     The  nucleus  is  generally  difficult 
to  detect :    it  is  often  hidden  in  a  clump  of  chloroplasts.     When 
visible  it  is  seen  as  a  rather  large,  pale,  often  faintly  granular  oval 
body.     Note  that  all  the  chloroplasts  are  embedded  in  cytoplasm  : 
none  of  them  lie  in  the  large  central  vacuole.      The  cytoplasm 
may  sometimes  be  seen  streaming  actively  round  the  cells,  carrying 
the  chloroplasts  with  it.     (Cf.  p.  85.) 


126  THE  GREEN  PLANT  CELL 

(4)  Mount  a  cross  section  of  the  fresh  stem  of  Pellionia  in  a 
drop  of  water.     Notice  under  the  high  power  the  chloroplasts 
in  the  cells  near  the  surface  of  the  section,  with  starch  grains  (bright 
colourless   granules)    inside   them.     In   cells    further   from   the 
surface  note  that  the  starch  grains  are  larger  and  have  burst 
out  of  the  chloroplasts.     The  chloroplast  which  has  formed  the 
grain  can  often  be  seen  attached  to  one  end  of  the  grain.     The 
growth  of  these  deeper  lying  grains  is  continuous  and  is  pro- 
bably the   result   of   a   continuous  supply  of  sugar,  while   the 
sugar  supply  of  the  superficial  cells  probably  depends,  as  in  a 
mesophyll  cell,  directly  upon  photosynthesis  and  therefore  upon 
light,  so  that  the  starch  grain  ceases  to  grow  and  is  converted 
back  into  sugar  every  night.      Draw  examples  under  the  high 
power  of  different  relations  of  starch  grain  and  chloroplast,  and 
then  test  with  iodine. 

(5)  With  the  hand  spectroscope  compare  the  spectrum  of  the 
alcoholic  solution  of  chlorophyll  (i)  with  that  of  the  translucent 
green  leaf  and  with  the  spectrum  of  sky  light.     [Compare  also 
with  the  demonstration  spectra.]     Note  the  absorption  bands  of 
the  chlorophyll  spectrum. 

(6)  Examine  the  demonstration  water  cultures,   and  observe 
the  effects  of  leaving  out  each  of  the  essential  elements  in  the 
mixed  solution  of  salts  used  to  feed  the  plant. 


CHAPTER   VIII 

THE  COLOURLESS  PLANT  CELL.  THE  YEAST 
PLANT 

WE  have  already  seen  that  many  of  the  living  cells 
of  the  higher  green  plants  do  not  contain  chlorophyll, 
e.g.  the  meristematic  cells,  all  the  cells  of  the  root, 
many  of  those  in  the  interior  of  the  stem,  and  also 
those  which  form  the  surface  layer  (epidermis)  of  her- 
baceous stems  and  leaves.  The  cytoplasm  of  many  of 
these  cells,  however,  contain  plastids,  which  may  turn 
green  on  exposure  to  bright  light,  and  thus  become 
chloroplasts.  All  these  colourless  cells  have  to  be  fed 
with  sugar,  ultimately  coming  from  the  green  cells, 
to  repair  the  waste  of  respiration  ;  and  the  meriste- 
matic cells,  which  are  actively  dividing,  not  only 
require  large  quantities  of  sugar,  but  must  also  be 
supplied  with  nitrogenous  substances  to  make  new 
protoplasm. 

There  are  also  plants  which  have  no  green  cells  : 
a  few  saprophytes  and  parasites  *  among  the  seed  plants 
which  live  upon  the  organic  substances  of  humus 
(decaying  vegetable  substance  such  as  leaf  mould) 
or  upon  the  organic  substances  of  the  bodies  of  living 
green  plants  ;  the  great  group  of  FUNGI  ;  and  finally 
the  group  of  unicellular  plants  known  as  BACTERIA, 
which,  as  we  shall  see  in  the  next  chapter,  are  of  extra- 
ordinary importance  in  the  economy  of  the  life  of  the 

1  See  p.  300. 
127 


128        THE   COLOURLESS   PLANT  CELL.      THE   YEAST  PLANT 

world.  All  of  these  colourless  plants,  with  the  excep- 
tion of  a  few  kinds  of  bacteria,  obtain  the  materials 
for  the  formation  of  new  protoplasm  and  the  energy 
for  carrying  out  their  life  processes  by  absorbing  liquid 
organic  substances  from  outside. 

Yeast  (Saccharomyces  J).— A  simple  organism  with 
which  it  is  convenient  to  begin  the  study  of  colourless 
plants  is  the  unicellular  yeast  plant.  There  are  many 
different  kinds  or  species  of  yeast,  some  of  them  differ- 
ing markedly  in  the  appearance  and  size  of  the  cell, 
but  mainly  distinguished  by  the  difference  of  their 
activities.  There  is  convincing  evidence  that  the 
yeasts  are  derived  from  a  certain  group  of  the  higher 
fungi,  but  they  live  and  maintain  themselves  indefinitely 
as  unicellular  plants. 

Structure  (Fig.  14). — The  single  yeast  cell  is  spherical 
or  oval  in  shape,  about  8  to  12  p  in  diameter — a  small 
cell  on  the  scale  of  the  tissue  cells  of  the  higher  plants, 
but  about  the  same  size  as  a  cell  of  Protococcus — sur- 
rounded by  a  cellulose  cell  wall.  The  cytoplasm  is 
more  or  less  granular,  and  there  is  a  central  oval  vacuole, 
though  the  diameter  of  this  in  proportion  to  that  of 
the  whole  cell  is  not  so  great  as  in  the  ordinary  adult 
tissue  cell  of  the  higher  plant — in  other  words,  the 
cytoplasmic  layer  lining  the  cell  wall  is  relatively 
thick  (Fig.  14,  C).  The  central  vacuole  is  really  intra- 
nuclear— fine  threads  of  chromatin  applied  to  the 
inside  of  nuclear  membrane  extending  around  the 
vacuole  from  an  aggregation  of  chromatin  at  one  end 
(Fig.  14  C,  chr.).  This  aggregation  of  chromatin  can 
sometimes  be  seen  as  a  granule  in  the  living  cell. 
According  to  the  metabolic  condition  of  the  cell,  other 
small  vacuoles  or  droplets  of  liquid,  and  granules  of 

1  "  Sugar-fungus." 


STRUCTURE   OF   YEAST   PLANT 


129 


various   substances,  may  appear   in    the   surrounding 
cytoplasm  (Fig.  14,  C). 
Conditions   of  Life. — "  Wild  "   yeasts   are   found  in 


FIG.  14. — Yeast  plant  (Saccharomyces) .  A,  yeast  cells  budding  and 
forming  chains,  x  200.  c.w.,  cell  wall ;  ppm.,  protoplasm ;  vac., 
vacuole.  B,  two  cells  forming  buds.  The  black  granules  are 
volutin,  a  complex  organic  substance  formed  by  the  yeast  cell. 

C,  diagram  of  the  structure  of  a  yeast  cell  made  up  from  informa- 
tion about  its  structure  obtained  from  treating  and  staining  the 
cell  with  various  reagents;  n.v.,  nuclear  vacuole;  «/.,  nucleolus ; 
chr.,    chromatin;    gl.,    glycogen    (after  Wager    and    Penistone). 

D,  formation  of  four  spores  within  a  cell. 

exposed    sugary    secretions    or    exudations    of    plants, 

for  instance  in  the  "  nectar  "  of  flowers,  in  the  drops 

9 


130   THE  COLOURLESS  PLANT  CELL.   THE  YEAST  PLANT 

of  juice  appearing  on  the  surface  of  a  fruit  with  ruptured 
skin,  and  the  like.  But  the  yeasts,  like  the  wheat  plant 
or  the  apple  tree,  are  best  known  in  the  "  domesticated  " 
or  cultivated  condition.  There  are  two  main  uses  to 
which  yeast  is  put  by  man — the  fermentation  of  sugary 
plant  juices  to  make  alcoholic  drinks,  such  as  beer, 
wine,  or  cider,  and  the  "  raising  "  of  bread  by  means 
of  the  carbon  dioxide  given  off  by  active  yeast. 

"  Brewers'  yeast "  is  a  frothy,  viscous,  cream- 
coloured  liquid  with  an  "  alcoholic  "  smell.  "  Bakers' 
yeast "  is  a  similarly  coloured  putty-like  substance. 
A  little  of  either  placed  in  a  drop  of  water  under  the 
microscope  is  seen  to  consist  of  myriads  of  yeast  cells, 
but  while  in  bakers'  yeast  these  are  mostly  single,  in 
brewers'  yeast  they  are  united  in  chains  which  are  often 
branched,  the  result  of  rapid  reproduction  by  budding 
(Fig.  14,  A  and  B). 

Nutrition. — Yeast  can  live  in  a  culture  solution  of 
salts  containing  the  essential  elements  of  the  food  of 
protoplasm,  of  which  the  most  complex  is  ammonium 
tartrate  (NH4)2C4H4O6,  an  organic  salt  containing 
carbon,  hydrogen  and  oxygen  as  well  as  nitrogen.  The 
other  elements  can  be  obtained  from  simple  inorganic 
salts  such  as  potassium  phosphate,  K3P04,  calcium 
phosphate,  Ca3(PO4)2,  and  magnesium  sulphate,  MgSO4. 
With  these  alone,  therefore,  it  can  construct  proteins. 
But  it  grows  very  slowly  in  such  a  medium,  for  the 
amount  of  energy  it  can  obtain  from  ammonium 
tartrate  is  small.  If  sugar  be  added  to  the  solution 
it  grows  and  multiplies  much  faster,  for  from  the  sugar 
it  can  obtain  a  large  amount  of  energy. 

Reproduction. — Yeast  reproduces  itself  primarily  by 
budding  (Fig.  14,  A  and  B) .  A  tiny  area  of  the  cell  wall, 
usually  at  one  end  of  the  oval  cell,  is  thrust  out  by  the 


BUDDING   AND   SPORE   FORMATION  13! 

osmotic  pressure  within  the  cell,  cytoplasm  passing 
into  the  bud.  The  bud  grows  until  it  reaches  the  size 
of  the  mother  cell,  the  nucleus  of  which  meanwhile 
divides,  one  of  the  daughter  nuclei  passing  into  the 
new  cell.  The  daughter  cell  may  then  be  cut  off  and 
become  a  new  unicellular  yeast  plant.  So  actively, 
however,  does  budding  take  place  in  a  solution  contain- 
ing sugar  (with  the  other  necessary  elements  present 
in  a  suitable  form)  that  the  daughter  cell  produces 
another  bud  before  separation  has  taken  place,  and 
this  again  another,  and  thus  the  chains  of  yeast  cells 
are  formed.  A  cell  may  even  bud  in  two  or  three 
places  at  once,  so  that  the  chains  are  branched. 

Spore  Formation. — Another  method  of  reproduction 
may  occur,  however,  if  the  yeast  is  starved,  e.g.  by  being 
left  on  a  slab  of  plaster  of  Paris  with  a  little  water. 
Under  these  circumstances  the  nucleus  divides  into 
two  and  then  into  four,  the  cytoplasm  withdraws  from 
the  wall  and  becomes  aggregated  round  the  four  neclei 
in  four  tiny  spherical  masses,  each  of  which  secretes  a 
comparatively  thick  wall  (Fig.  14,  D).  Each  of  the 
spores  so  formed  is  about  3  \i  in  diameter.  If  now  the 
yeast  culture  dries  up,  the  original  cell  wall  collapses, 
and  the  spores  can  be  carried  away  and  float  with 
the  dust  in  the  air,  the  living  contents  remaining  in 
a  dormant  condition  for  a  long  time.  Falling  into  a 
suitable  liquid  medium,  the  dormant  protoplasm  of 
the  spore  absorbs  the  liquid  and  becomes  active,  burst- 
ing the  spore  wall,  pushing  out,  and  covering  itself 
with  a  new  ordinary  cell  wall,  thus  giving  rise  to  a  new 
vegetative  yeast  cell. 

The  spores  of  yeast  are  therefore  a  resting  stage  in  the 
life  history,  able  to  withstand  desiccation  for  some  time. 
This  is  true  of  most  of  the  spores  formed  by  plants. 


132   THE  COLOURLESS  PLANT  CELL.   THE  YEAST  PLANT 

Alcoholic  Fermentation. — The  yeast  plant  is  of 
special  interest  both  biologically  and  practically,  because 
it  is  able  for  a  time,  in  a  suitable  nutritive  medium 
containing  sugar,  to  carry  on  a  process  which  we  must 
regard  as  a  modified  kind  of  respiration,  in  place  of 
ordinary  respiration.  The  yeast  cells  produce  an 
enzyme  (zymase},  by  the  catalytic  activity  of  which 
the  molecule  of  sugar  (glucose)  is  split  into  two  mole- 
cules of  ethyl  alcohol  and  two  of  carbon  dioxide  : — 

C6H12O6  =  2C2H6O  +  2CO2 

glucose        ethyl  alcohol    carbon 
dioxide 

thus  liberating  energy  from  the  sugar  molecule 
without  using  free  oxygen.  This  process  in  the  living 
cell  is  called  anaerobic  respiration,  as  opposed  to  the 
ordinary  aerobic  J  respiration,  in  which  free  oxygen  is 
used.  Of  the  sugar  actually  absorbed  by  the  yeast 
cells  some  is  split  up  in  this  way  and  the  energy  is 
used  in  growth,  while  some  is  doubtless  used,  together 
with  nitrogenous  substances,  for  the  formation  of  new 
protoplasm.  None  of  the  zymase  formed  by  the 
cell  passes  out  into  the  surrounding  sugar  solution, 
but  quantities  of  sugar  pass  into  the  yeast  cells,  and 
are  split  up  into  alcohol  and  carbon  dioxide.  The 
carbon  dioxide  forms  bubbles  in  the  liquid,  which  froths, 
while  the  alcohol  accumulates,  and  the  energy  set 
free  raises  the  temperature  of  the  liquid.  Meantime 
the  yeast  cells  grow  and  bud  with  great  rapidity,  enor- 
mously increasing  their  number.  This  is  the  process 
known  as  alcoholic  fermentation,  and  is  the  foundation 
of  the  making  of  beer  and  wine. 

Brewing. — The  liquid  fermented  in  brewing  beer  is 
called  wort,  and  is  a  hot  water  infusion  of  malt,  i.e.  sprout- 
!  "  Living  in  air  "  from  Greek  dijp,  air,  and  /St'og,  life. 


MALTING   BARLEY  133 

ing  barley  grains.  Water  is  added  to  the  fresh  barley 
grains  and  kept  standing  on  them  about  three  days, 
being  changed  two  or  three  times  during  that  period. 
The  wet  grains  are  then  spread  out  on  the  floor  of  the 
malting  house,  and  occasionally  raked  over  so  as  to 
allow  the  air  free  access  to  them.  The  temperature 
of  the  grains  rises  considerably  during  the  week  or  so 
they  are  left  on  the  floor.  This  is  due  to  the  production 
of  heat  by  the  respiration  of  the  grains,  which  absorb 
oxygen  from  the  air.  The  starch  stored  in  the  grains 
is  converted  into  sugar,  and  much  of  this  sugar  is  split 
up  in  respiration.  At  the  end  of  the  "  malting,"  however, 
the  sprouting  grains  are  still  full  of  sugar  and  of  soluble 
nitrogenous  organic  substances — the  reserve  stores  of 
the  seed  are  mobilised  for  the  growth  of  the  young  plant. 
The  malt  is  then  removed  to  kilns  and  dried  for 
one  or  two  days  at  70°  or  100°  C.,  according  to  the  kind 
of  beer  that  is  to  be  produced.1  The  kilned  malt  is 
removed  to  vats  to  which  water  is  added  at  50°  C.,  the 
temperature  being  afterwards  raised  to  76°  C.  This  is 
the  process  of  infusion  of  the  malt,  the  liquid  produced 
being  the  wort,  which  is  now  filtered  off,  the  malt 
residue,  which  still  contains  a  considerable  amount  of 
nutritive  substance,  being  used  as  cattle  food.  The  wort 
is  boiled  with  hops  for  two  or  three  hours  to  extract  the 
bitter  aromatic  substances  from  the  hops,  and  then 
sieved  off  and  run  into  underground  marble  vats,  where 
it  is  cooled  to  8°  C.,  and  the  yeast — a  pure  culture  of  a 
particular  strain  of  yeast 2 — is  added.  Here  it  stands 

1  The  details  of  malting  and  brewing  procedure  given  are  those 
followed  at  the  famous  Carlsberg  Brewery  at  Copenhagen,  where 
"  Pilsner  "  and  "  Lager  "  light  beers  are  the  products.  The  malt  is 
kilned  at  70°  C.  for  the  former  and  at  100°  C.  for  the  latter. 

»  In  the  case  of  "  Pilsner  "  and  "  Lager  "  beer  a  so-called  "  bottom 
yeast,"  which  sinks  to  the  bottom  of  the  wort.  For  brewing  the 
heavier  English  beers  "  top  yeasts  "  are  used. 


134   THE  COLOURLESS  PLANT  CELL.   THE  YEAST  PLANT 

for  twelve  to  fourteen  days  while  the  process  of  fermenta- 
tion takes  place.  The  carbon  dioxide  produced  rises  to 
the  top,  so  that  the  surface  of  the  fermenting  wort 
becomes  covered  with  a  thick  layer  of  froth.  Much  of 
the  carbon  dioxide  escapes,  but  being  heavier  than  air 
lies  on  the  surface  of  the  vat,  so  that  workmen  have 
sometimes  been  asphyxiated  by  approaching  the  vats 
incautiously.  The  temperature  of  the  wort  rises,  owing 
to  the  great  liberation  of  energy  in  fermentation,  but 
is  kept  down  to  12°  C.  by  currents  of  cold  air  directed 
on  to  the  vats.  When  the  fermentation  has  reached 
the  desired  point  the  beer  is  run  into  specially  lined 
storage  casks  or  tanks  and  kept  at  i°  C,  for  three  months, 
when  it  is  ready  for  bottling. 

The  whole  of  the  installation  is  kept  scrupulously 
clean,  or  infection  of  the  wort  with  "  wild  "  yeasts  and 
bacteria  would  occur,  undesired  fermentations  would 
take  place,  and  the  beer  spoiled. 

The  kind  and  quality  of  beer  produced  depends  not 
only  on  the  kind  of  yeast  used  and  the  time  during 
which  fermentation  proceeds,  but  also  on  the  kind  and 
quality  of  the  malt  and  hops  (and  these  again  on  the 
strains  of  barley  and  hops,  and  on  the  soil  and  climate 
in  which  they  are  grown),  for  beer  is  a  complex  solution 
of  many  substances  derived  from  different  bodies  con- 
tained in  the  malt  and  hops,  besides  the  sugar,  which 
is  the  main  substance  fermented. 

Wine  -  Making. — In  wine-making  the  fermentable 
liquid  (must)  is  grape  juice,  which  is  pressed  from  the 
overripe  grapes  in  a  wine-press.  The  yeasts  which 
cause  the  fermentation  in  the  must  are  always  present 
on  the  overripe  grapes,  and  do  not  have  to  be  added 
as  is  done  in  the  making  of  beer.  As  in  the  case  of 
beer,  but  to  a  far  higher  degree,  the  kind  and  quality 


VINTAGE  WINES.      FERMENTATION  AND  RESPIRATION      135 

of  wine  depends  not  only  on  the  way  it  is  made,  but 
on  the  kind  of  grape  used,  the  climate  and  soil  in  which 
the  vines  are  grown,  and  the  weather  during  the  season 
in  which  the  grapes  ripen.  Thus  not  only  are  claret, 
port,  sherry  and  hock,  for  instance,  totally  different 
wines,  because  they  are  made  in  different  ways,  but 
particular  vineyards  in  each  wine-growing  district 
produce  particular  qualities  of  wine,  and  finally  the 
different  years  or  "  vintages  "  from  the  same  vineyard 
have  different  characteristics  owing  to  the  weather 
during  the  year.  Sometimes  the  characters  so  pro- 
duced actually  override  the  differences  due  to  the 
different  vineyards,  so  that  a  connoisseur  can  recognise 
a  particular  vintage.  A  fine  wine  is  an  immensely 
complex  solution,  and  the  chemistry  of  many  of  the 
organic  substances  it  contains  is  still  very  imperfectly 
understood.  Slow  chemical  changes  continue  in  the 
wine  while  it  is  in  cask  and  while  it  is  in  bottle.  Up 
to  a  certain  point  the  wine  constantly  "  improves," 
and  after  that  point  deteriorates. 

In  making  sparkling  wines  the  later  stages  of  fer- 
mentation are  allowed  to  take  place  in  the  bottles,  and 
the  carbon  dioxide,  which  develops  a  considerable  pres- 
sure, is  thus  retained.  Fermentation  is,  however,  always 
stopped  before  it  is  complete  and  the  yeast  removed. 

Relation  of  Fermentation  to  Respiration.  —  The 
alcoholic  fermentation  of  sugar  by  the  yeast  plant,  or 
rather  by  the  enzyme  zymase  which  it  produces,  is,  as 
has  been  said,  to  be  regarded  as  a  modified  kind  of 
respiration  into  which  free  oxygen  does  not  enter 
(anaerobic  respiration).  But  yeast  cannot  go  on  fer- 
menting sugar  indefinitely.  After  a  time  it  must  have 
free  oxygen  and  respire  aerobically.  It  is  interesting 
to  note  that  ordinary  (aerobic)  respiration  begins  with 


136   THE  COLOURLESS  PLANT  CELL.   THE  YEAST  PLANT 

a  splitting  of  the  sugar  molecule  without  oxidation, 
free  oxygen  only  taking  part  in  the  reaction  towards 
the  end  of  the  process,  resulting  in  the  formation  of 
carbon  dioxide  and  water.  Alcohol  may  actually  be 
formed  in  ordinary  plant  cells  if  free  oxygen  is  ex- 
cluded. Thus  alcoholic  fermentation  by  yeast  repre- 
sents the  first  stage  of  the  ordinary  respiratory  process 
continued  for  a  long  time  and  very  energetically. 

Alcoholic  fermentation  of  sugar  by  yeast  is  only 
one  kind  of  fermentation.  Sugar  can  be  split  up  into 
other  substances  than  ethyl  alcohol  and  carbon  dioxide, 
and  a  great  variety  of  other  organic  substances  can  be 
split  up  in  a  similar  way.  These  varied  fermentations 
are  mainly  carried  out  by  various  members  of  the  great 
class  of  unicellular  plants,  the  Bacteria,  with  which  we 
shall  deal  in  the  next  chapter. 

PRACTICAL  WORK. 
A.  YEAST. 

(1)  The  corked  bottle  contains  yeast  fermenting  a  sugar  solu- 
tion.    Dip  a  glass  rod  into  lime-water  and  test  the  air  in  the 
bottle  for  carbon  dioxide  in  the  same  way  as  with  the  respiring 
barley  (p.  89).     Repeat  the  test  at  the  end  of  the  practical  work. 

(2)  Place  a  drop  of  brewers'  yeast  on  a  slide,  cover,  and  examine 
with  the  high  power.     Measure  the  length  and  breadth  of  several 
of  the  isolated  cells  and  determine  their  average  size.     Draw  two 
or  three  on  a  large  scale,  marking  cell  wall,  cytoplasm  and  central 
(nuclear)  vacuole.      A  refractive  mass  (chromatin)  can  often  be 
seen  at  one  end  of  the  vacuole.     Note  also  the  granules  and 
small  vacuoles  that  can  often  be  seen  in  the  substance  of  the 
cytoplasm.     Stain  with  Schulze's  solution.     The  cell  wall  stains 
blue  (cellulose). 

(3)  Trace  the  method  of  budding  by  which  the  yeast  cells 
multiply  and  draw  examples  of  different  stages. 

(4)  Smear  a  drop  of  yeast  culture  thinly  over  a  perfectly 
clean  coverslip  and  dry  the  smear  over  the  flame  of  a  lighted 
match.     Place  five  drops  of  methylene  blue  in  a  clean  watch- 
glass  and  fill  it  up  with  water.     Immerse  the  coverslip  in  the 


PRACTICAL   WORK  137 

stain  and  leave  it  there  for  one  minute.  Take  it  out  and  wash 
by  gently  moving  the  coverslip  in  water.  Dry  both  sides  of 
the  coverslip  with  blotting  paper.  Find  out  on  which  side  the 
smear  is,  and  place  the  coverslip,  smear  downwards,  on  a  small 
drop  of  dilute  glycerine  on  a  slide.  Examine  with  the  high 
power. 

B.  THICK  CELLULOSE  AND  MUCILAGINOUS  CELL  WALLS. 

(1)  Examine  in  water  sections  of  the  date-stone  (seed  of  the 
date-palm)  cut  so  as  to  show  the  elongated  cells  in  transverse 
section.     Note  the  very  thick  cell  walls.     These  are  penetrated 
by  pits  running  from  each  cell  cavity  and  ending  at  the  middle 
lamella  of  the  cell  wall   (the  middle  lamella  was  the  original 
thin  wall  separating  two  adjacent  cells  before  thickening  took 
place).     The  pits  of  adjacent  cells  are  always  exactly  opposite 
one  another. 

Mount  the  section  in  a  drop  of  Schulze's  solution,  and  leave 
for  ten  minutes.  Then  note  that  the  mass  of  the  cell  wall  is 
stained  purple  (cellulose),  while  the  middle  lamella  remains 
unstained  or  only  faintly  stained.  The  mass  of  cellulose  is 
converted  into  sugar  when  the  seed  germinates  and  the  sugar  is 
used  for  the  respiration  and  growth  of  the  young  plant. 

(2)  Examine  a  dry  seed  of  the  flax  plant   (linseed).     Note 
the  sharp  outline  of  the  opaque  seed.     Now  put  the  seed  in  a 
watchglass  with  a  little  water.     The  outline  of  the  seed  is  soon 
surrounded  by  a  transparent  fringe.     Compare  the  feel  of  the 
dry  and  the  wet  seed  between  the  fingers. 

Now  examine  a  thin  section  of  the  seed  dry  under  a  coverslip, 
and,  running  in  a  drop  of  water,  watch  the  swelling  of  the  surface 
layer  of  cells.  The  middle  lamellae  of  the  walls  separating  the 
cells  can  be  distinguished  from  the  swollen  mucilaginous  mass 
of  the  walls.  Note  also  the  cuticle  bounding  the  walls  on  the 
outside.  Methylene  blue  will  stain  the  mucilage  and  render  it 
more  easily  visible. 


CHAPTER    IX 
BACTERIA 

BACTERIA  are  the  smallest  of  known  organisms,  but 
they  are  of  quite  extraordinary  importance  in  the 
economy  of  nature,  an  importance  only  exceeded  by 
the  green  plants.  While  the  latter  form  the  basis  of 
all  life  by  building  up  living  substance  from  inorganic 
materials,  the  former  destroy  dead  organic  matter, 
breaking  it  down  into  simpler  forms  and  eventually 
into  inorganic  substances  which  are  available  for  the 
food  of  green  plants.  Bacteria  are  also  of  direct  prac- 
tical importance  to  men,  not  only  because  some  of  them 
cause  various  deadly  diseases,  but  because  others  are 
used  by  him,  as  yeast  is  used,  to  carry  out  fermentations 
— not  alcoholic  fermentations;  but  processes  of  the  same 
general  nature,  such  as  the  "  ripening  "  of  cheese,  the 
"  curing  "  of  tobacco,  and  so  on.  There  are  a  large 
number  of  different  species  of  bacteria  known,  and 
certainly  far  more  exist  than  have  yet  been  recognised. 
Size,  Shape  and  Structure. — The  diameter  of  an 
average  bacterial  cell  is  only  about  I  p,,  far  less  than 
that  of  other  unicellular  organisms.  There  certainly 
exist,  however,  living  organisms  which  may  probably 
belong  to  the  Bacteria  and  which  are  too  small  to  recog- 
nise under  the  highest  powers  of  the  microscope.1  Thus 
we  know  that  yellow  fever,  and  also  swine  fever,  are 

1  It  will  be  remembered  that  an  object  less  than  about  -15^  in 
diameter  cannot  be  clearly  denned  under  the  highest  powers  of  the 
microscope  (p.  50). 

138 


FORM  TYPES  OF  BACTERIA  139 

caused  by  different  specific  living  organisms  which  are 
invisible  and  which  will  pass  through  the  meshes  of  a 
porcelain  filter  that  excludes  any  visible  bacterium.  It 
is  at  least  probable  that  many  different  kinds  of  such 
ultramicroscopic  organisms  exist,  for  there  is  room 
between  the  lower  limit  of  microscopic  visibility  and 
the  size  of  a  complex  protein  molecule  for  a  collection 
of  such  molecules  to  be  associated  into  a  system  which 
would  form  an  ultramicroscopic  protoplasmic  unit  with 
specific  characters. 

The  visible  bacteria  may  be  classed  in  two  groups — 
a  group  of  simpler  forms  which  are  the  more  numerous, 
and  a  more  highly  developed  group  with  which  we  need 
not  concern  ourselves  here.  The  simpler  forms  may 
be  roughly  classed  according  to  their  shape  into  three 
types :  (i)  the  coccus,  which  is  spherical,  on  the  average 
i  /x  in  diameter  ;  (2)  the  bacillus,  a  straight  cylindrical 
rod,  i  /A  or  less  in  diameter  and  up  to  about  10  /u,  long  ; 
and  (3)  the  spirillum,  a  curved  rod  which  may  be  spirally 
coiled  like  a  corkscrew.  Some  bacteria,  however,  are 
not  of  these  shapes,  but  may  be  oval  in  outline,  etc. 
(see  Fig.  15).  When  bacteria  were  first  beginning  to 
be  systematically  studied,  sixty  or  seventy  years  ago, 
these  "  form  types"  were  largely  used  as  generic  names, 
the  different  species  being  distinguished  according 
to  the  habitat  or  activity,  and  some  of  these  species 
are  still  recognised,  e.g.  Bacillus  coli  (which  lives  in 
the  human  intestine),  Bacillus  anthracis  (which  causes 
the  disease  anthrax),  and  so  on.  It  is  not,  however,  the 
form  of  the  bacterial  cell  but  its  specific  activity 
that  is  its  most  important  feature,  and  many  new 
genera  have  been  established. 

Each  bacterial  cell  consists  of  a  minute  mass  of 
protoplasm  in  which  a  separate  nucleus  cannot  be 


140 


BACTERIA 


detected.  The  whole  of  the  protoplasm  stains  strongly 
with  the  ordinary  nuclear  stains,  and  it  is  likely  that 
what  corresponds  with  the  chromatin  of  the  ordinary 


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FIG.  15. — Various  forms  of  Bacteria,  x  1,000.  A,  Staphylococci 
(groups)  and  Streptococci  (curved  chains)  from  pus  (the  large 
spherical  body  is  a  pus  corpuscle).  B,  Nitrosomonas,  a  nitrifying 
bacterium  from  soil,  with  single  flagellum.  C,  Bacillus  typhosus 
(typhoid),  showing  flagella.  D.  Bacillus  anthracis  (anthrax), 
showing  spores  (one  in  the  centre  of  each  cell).  E,  Bacillus 
tetani  (tetanus).  Note  the  spherical  spore  at  one  end  of  and 
much  broader  than  the  cell.  F,  Cholera  spirillum,  with  a  flagellum 
at  each  end  of  the  cell.  G,  Spirochcete  pallida  (syphilis  spirillum). 
After  Muir  and  Ritchie. 

cell  nucleus  exists  in  the  bacterial  cell  equally  distri- 
buted through  the  cytoplasm.  The  protoplasm  is 
surrounded  by  a  membrane  of  gel  structure  (as  is  always 


MOVEMENT  14! 

the  case  on  the  surface  of  a  protoplasmic  unit).  This 
is  not  composed  of  cellulose,  and  is  not  to  be  compared 
with  the  cell  wall  of  an  ordinary  plant  cell.  In  some 
kinds  of  bacteria  the  colloid  membrane  swells  very 
greatly  in  the  liquid  medium  inhabited  by  the  organism, 
and  thus  the  individual  cell,  or  group  or  colony  of 
cells,  may  become  surrounded  by  a  thick  mucilaginous 
investment,  making  the  whole  colony  quite  visible  to 
the  naked  eye  and  slimy  to  the  touch.  A  colony  of 
this  kind  is  called  a  zooglcea. 

Flagella  or  cilia  (extremely  delicate  threads  of  proto- 
plasm arising  from  the  surface  of  the  cell,  singly  or  in 
groups,  and  projecting  into  the  surrounding  medium) 
may  be  detected  in  many  species  by  special  methods 
of  staining  (Fig.  15,  B,  C,  F). 

Movement  and  Response  to  Stimuli. — Many  kinds 
of  bacteria  swim  about  actively  in  a  liquid  medium, 
in  most  cases  no  doubt  by  the  beating  of  their  flagella, 
for  these  have  never  been  detected  in  the  non-motile 
forms.  Some  bacteria,  however,  especially  the  spirilla, 
appear  to  move  by  wriggling  their  bodies  (rather  as 
an  eel  moves),  i.e.  by  the  contractility  of  the  cell  proto- 
plasm as  a  whole. 

Motile  bacteria  move,  as  a  rule,  in  response  to  chemical 
stimuli,  i.e.  towards  some  substances  and  away  from 
others  (positive  and  negative  chemotaxis).  A  good 
example  is  Bacterium  termo,  mentioned  on  p.  113  as 
specially  sensitive  to  free  oxygen  and  used  in  Engel- 
mann's  experiment  to  pick  out  the  particular  rays  of 
the  spectrum  in  which  the  chloroplasts  liberate  most 
oxygen. 

Nutrition. — The  foods  of  bacteria  are  very  various 
both  in  kind  ant1  in  chemical  nature,  and  their  habitats 
naturally  correspond  with  these  different  foods.  Some 


142  BACTERIA 

species  feed  directly  on  the  proteins  of  living  animals 
or  plants,  some  on  various  solid  or  liquid  organic  sub- 
stances in  the  living  body  or  outside  it.  Some  can 
get  their  nitrogen  from  comparatively  simple  organic 
salts  like  ammonium  tartrate,  just  as  yeast  can  ; 
others  get  it  from  even  simpler  salts  ;  others,  again,  can 
use  the  free  nitrogen  of  the  air.  A  few  can  form 
their  protoplasm  from  inorganic  substances  alone,  as 
green  plants  can,  though  they  do  it  in  quite  a  different 
way,  using  the  energy  liberated  in  the  oxidation  process 
instead  of  the  energy  of  sunlight.  The  great  majority, 
however,  feed  on  more  or  less  complex  organic  sub- 
stances, from  which  they  obtain  energy  for  growth  and 
multiplication. 

The  actual  form  in  which  the  food  is  absorbed  by 
the  bacterial  cell  must  of  course  be  liquid  or  gaseous 
— in  the  great  majority  of  cases  it  is  liquid.  That  is 
why  we  class  bacteria  as  plants,  and,  so  far  as  their 
nutrition  is  concerned,  with  the  great  group  of  colour- 
less plants  known  as  the  fungi.  But  both  in  structure 
and  affinities  (by  which  we  mean  descent)  the  bacteria 
are  very  different  from  and  most  probably  have  no 
connexion  with  the  fungi  proper.  Bacteria  are  most 
probably  more  or  less  directly  derived  from  the  most 
primitive  forms  of  life,  of  which  we  have  no  direct 
knowledge.  We  must,  however,  conceive  of  primitive 
organisms  as  consisting  of  very  minute  forms  living  in 
water,  and  obtaining  their  energy  in  different  ways — 
from  light,  from  chemical  energy  such  as  that  liberated 
in  the  oxidation  process,  or  from  ingesting  other  minute 
organisms.  From  this  undifferentiated  group  the  green 
plants  developed  along  one  line  ;  the  animals,  with  their 
habit  of  feeding  on  other  organisms,  along  another  ; 
while  the  bacteria  as  we  know  them  to-day  are  a  highly 


REPRODUCTION   AND    SPORE   FORMATION  143 

specialised  group,  most  of  which  feed  on  liquid  organic 
substance,  while  retaining  an  exceedingly  simple  struc- 
ture. Bacteria  in  the  same  forms  shown  by  existing 
species  have  been  found  preserved  in  fossils  many 
millions  of  years  old. 

Reproduction. — The  simpler  bacteria  multiply  by 
simple  division  of  the  cell  and  separation  of  the  daughter 
cells  so  formed.  Sometimes  the  cell  grows  to  a  larger 
size  than  the  normal  before  division  ;  in  other  cases 
the  cell  divides  and  each  daughter  cell  subsequently 
grows  to  the  normal  size.  The  daughter  cells  of  a  bacillus 
frequently  remain  together  for  a  time  after  division,  so 
that  lines  of  cells  having  the  form  of  jointed  rods  are 
produced. 

In  most  bacteria  growth  and  multiplication  proceed 
with  great  rapidity.  Under  very  favourable  condi- 
tions a  bacterium  may  reach  maturity  and  divide  in 
twenty  to  thirty  minutes.  If  division  takes  place  only 
once  an  hour,  seventeen  million  individuals  will  have 
arisen  from  a  single  cell  in  twenty- four  hours.  The  chief 
factors  which  control  the  rate  of  multiplication  are 
temperature  and  food  supply. 

Spore  Formation. — In  certain  species  of  the  simpler 
bacteria,  chiefly  bacilli,  spores  are  produced  under 
certain  conditions.  These  spores  are  formed  in  the 
same  sort  of  way  as  yeast  spores,  by  the  aggregation 
of  the  protoplasm  at  a  given  spot  in  the  cell,  the  proto- 
plasm of  the  spore  becoming  surrounded  by  a  dense  wall. 
As  a  rule  only  one  spore  is  formed  in  each  cell,  either 
in  the  centre  or  close  to  one  end.  When  the  spore 
comes  into  conditions  suitable  for  growth,  the  spore  wall 
is  split,  and  a  new  vegetative  cell  protrudes,  assuming 
the  form  characteristic  of  the  species. 

Bacterial  spores  are  by  far  the  most  resistant  form 


144  BACTERIA 

of  living  protoplasm  known.  Many  can  withstand 
for  some  time  exposure  to  dry  heat  of  considerably 
over  100°  C,  while  most  vegetative  forms  are  quickly 
killed  at  50°  C.  In  some  cases  spores  can  even  with- 
stand immersion  in  boiling  water  for  several  minutes, 
while  they  may  remain  alive  in  a  dry  and  inactive 
condition  for  years.  The  great  difference  between  the 
powers  of  resistance  of  vegetative  cells  and  spores  is 
illustrated  by  the  anthrax  bacillus  (B.  anthracis). 
The  active  vegetative  cell  is  killed  by  two  minutes' 
exposure  to  I  per  cent,  carbolic  acid,  while  spores 
resist  immersion  in  the  same  strength  in  some  cases 
for  fifteen  days.  Vegetative  forms  can,  however,  with- 
stand extreme  cold  for  a  long  time,  for  instance  the 
temperature  of  liquid  air  (—  190°  C.)  for  twelve 
months. 

Dispersal. — Spore  formation  is  not  a  means  of  multi- 
plication in  bacteria  any  more  than  it  is  in  the  case  of 
yeast.  Only  one  spore  is  normally  formed  in  the  bac- 
terial cell,  and  the  growth  and  division  of  the  organism 
is  completely  arrested  until  the  spore  germinates. 
The  spore  condition  is  the  resting  stage  of  the  bacterium, 
enabling  it  to  withstand  conditions  unfavourable  for 
vegetative  growth  and  multiplication.  Spores  are,  of 
course,  an  important  means  of  dispersal  for  the  forms 
which  produce  them,  since  they  are  passively  carried 
about  in  currents  of  air  without  injury,  whereas  vegeta- 
tive bacterial  cells  will  sooner  or  later  be  killed  by  drying 
up.  But  only  comparatively  few  species  of  bacteria 
are  known  to  produce  spores,  most  kinds  being  dis- 
persed entirely  by  means  of  the  ordinary  cells.  Bac- 
teria often  get  into  the  air  by  detachment  of  minute 
particles  from  the  surface  on  which  they  are  growing. 
Such  particles  form  part  of  the  dust  of  the  air,  and  prac- 


HABITATS   AND    LIFE    CONDITIONS  145 

tically  all  air  at  low  levels,  except  over  mid-ocean, 
contains  more  or  less  dust.  Dust  is  carried  about  in 
currents  of  air,  and  is  often  blown  by  the  wind  for  very 
long  distances.  When  the  air  becomes  comparatively 
quiet,  the  particles  drift  slowly  down  and  settle.  Any 
living  cells  which  it  contains,  vegetative  bacterial 
cells,  bacterial  spores  or  fungal  spores,  will  grow  if 
they  find  themselves  in  suitable  conditions  of  moisture, 
food  supply  and  temperature.  In  such  conditions 
spores  will  germinate,  and  such  vegetative  cells  as  have 
not  been  killed  by  drying  up  will  grow  and  divide. 

Habitats  and  Conditions  of  Life.— This  is  why  all 
dead  bodies  exposed  to  air  putrefy  and  eventually  fall 
to  pieces,  provided  they  remain  moist  and  not  too 
cold  ;  and  why  all  milk,  beer  or  wine  similarly  exposed 
turns  sour.  These  processes  all  depend  on  the  activity 
of  the  living  protoplasm  of  bacteria.  Each  is  carried 
out  by  particular  species  only,  and  cells  or  spores  of 
some  of  these  are  always  found  in  the  dust  of  the  air, 
so  that  they  are  certain  to  fall  sooner  or  later  on  to  the 
surface  of  the  dead  body  or  into  the  organic  liquid. 
These  processes  go  on  more  rapidly  at  fairly  high 
temperatures,  simply  because  such  temperatures  are 
the  most  favourable  for  bacterial  growth  and  meta- 
bolism ;  and  they  are  suspended  at  o°  C.  or  below 
(though  the  bacteria  are  not  killed),  because  protoplasm 
cannot  work  at  the  freezing-point  of  water. 

In  order  to  feed,  live  and  multiply,  each  species  of 
bacterium  must  have  the  appropriate  conditions  for 
that  species,  the  right  sort  of  food,  the  presence  (or 
absence)  of  oxygen,  a  sufficiency  of  water  in  the  medium, 
and  a  suitable  temperature.  The  temperature  relations 
of  bacteria  are  remarkably  wide  and  various.  Most 
species  live  at  the  ordinary  temperatures  which  prevail 
10 


146  BACTERIA 

in  the  part  of  the  world  where  they  are.  In  temperate 
climates  few  species  will  grow  at  temperatures  above 
30°  C.,  though  some  occur  in  manure  heaps,  etc.,  whose 
optimum1  is  as  much  as  70°  C.  Species  which  are  para- 
sitic on  warm-blooded  animals  live  best  at  37°  C.,  and 
some  of  them  die  at  temperatures  less  than  20°  C. 

Sterilisation. — If  all  bacteria  and  their  spores  are 
to  be  destroyed  in  a  given  enclosed  space,  the  air  in 
that  space  may  be  raised  to  a  temperature  which  they 
cannot  survive.  This  can  usually  be  effected  by  dry 
heating  to  160°  C.  for  half  an  hour  or  to  180°  C.  for  ten 
minutes.  If  a  liquid  is  to  be  similarly  "  sterilised," 
it  may  be  boiled  in  a  flask  for  several  minutes,  with  a 
cotton  wool  plug  inserted  in  the  neck  of  the  flask. 
The  contents  of  the  flask  will  then  be  thoroughly  cleared 
of  active  cells.  The  plug  acts  as  a  filter  to  prevent  the 
entrance  of  cells  or  spores  during  cooling  when  air  is 
drawn  into  the  flask  through  contraction  of  the  cooling 
air  inside.  Spores  in  the  liquid  may,  however,  survive 
the  boiling.  Heating  with  steam  at  high  pressure 
(which  raises  the  temperature  of  the  steam)  at  115°  C. 
for  ten  minutes,  or  120°  for  five  minutes,  will  kill  any- 
thing, even  the  most  resistant  spores. 

A  flask  containing  a  fermentable  or  putrescible 
liquid,  i.e.  a  liquid  in  which  bacteria  can  grow  and 
bring  about  changes  in  the  chemical  composition, 
thus  plugged  and  sterilised,  can  be  kept  for  any  length 
of  time  without  change.  If  the  plug  is  removed  at 
any  time  fermentation  will  at  once  begin  and  the  charac- 
teristic bacteria  will  be  found  in  the  liquid.  This  is 
a  clinching  proof,  originally  due  to  the  great  French 
chemist  and  biologist  Pasteur,  that  all  such  changes 
are  due  to  the  activity  of  micro-organisms. 

1  Most  favourable  temperature. 


STERILISATION   AND    PURE   CULTURE  147 

An  alternative  method  of  sterilising  a  liquid  which 
avoids  the  use  of  heat,  and  has  also  the  advantage  of 
excluding  the  bodies  of  bacteria,  is  by  filtering  through 
unglazed  earthenware.  The  liquid  to  be  sterilised  is 
forced  under  pressure  through  the  filter,  whose  pores 
must  be  so  small  that  they  will  not  allow  the  smallest 
bacterium  to  pass.  To  be  efficient  such  a  filter  must 
be  made  of  the  finest  "  china  clay."  But  in  practice 
sterilisation  by  filtration  is  uncertain,  because  of  in- 
equalities in  the  size  of  the  pores  and  imperfections  in 
the  filter.  Such  filters  are  often  attached  to  domestic 
water  taps  to  make  sure  that  drinking  water  contains 
no  harmful  bacteria. 

Pure  Cultures. — In  the  early  days  of  the  study  of 
bacteria  it  was  impossible  for  the  observer  to  be  sure 
that  he  was  not  dealing  with  a  mixture  of  different 
kinds.  The  cells  in  a  living  culture  of  bacteria  might 
show  various  forms,  or  bring  about  various  effects 
on  the  culture  medium  from  time  to  time.  This  might 
be  due  to  the  powers  of  a  single  kind  of  bacterium, 
or  on  the  other  hand  to  the  gradual  disappearance  of 
one  species  and  the  multiplication  of  another  (which 
was  also  present)  as  the  chemical  constitution  of  the 
medium  was  altered  owing  to  the  effect  of  the  bacteria 
upon  it.  No  increase  of  accurate  knowledge  of  the 
different  kinds  of  bacteria  was  possible  under  these 
conditions. 

The  discovery  of  the  possibility  of  complete  sterilisa- 
tion of  a  medium  by  Pasteur  not  only  proved  that  all 
changes  of  the  nature  of  putrefaction  and  fermentation 
were  brought  about  by  living  organisms,  but  it  enabled 
perfectly  pure  cultures  of  a  single  kind  of  bacterium  to 
be  made  and  kept.  The  making  of  pure  cultures  for 
the  study  of  bacteria  was  introduced  about  the  year 


148  BACTERIA 

1880  by  the  great  German  bacteriologist,  Koch,  and  has 
been  an  indispensable  means  of  developing  the  vast 
modern  science  of  bacteriology.  The  various  technical 
methods  of  making  pure  cultures  are  based  on  the  fact 
that  if  a  small  quantity  of  a  liquid  containing  bacteria 
is  diluted  with  a  large  excess  of  sterile  water  and  the 
bacteria  evenly  distributed  through  it,  each  drop  of 
the  mixture  will  contain  only  a  few  or  even  a  single 
organism.  Such  a  drop  added  to  a  sterile  culture 
medium  will  probably  give  rise  to  a  growth  consisting 
of  one  sort  of  bacillus  only.  In  practice  it  is  more 
convenient  to  immobilise  the  bacteria  in  the  diluted 
mixture  by  making  it  with  a  warm  gelatine  or  agar 
jelly,  which  will  set  when  it  is  cold.  The  individuals 
then  multiply  and  give  rise  to  visible  "  colonies," 
each  originating  from  one  organism,  which  can  be  trans- 
ferred to  other  sterile  media,  and  the  forms  of  the  cells 
and  the  effects  of  their  growth  on  the  medium  can  be 
studied  at  leisure. 

Genera,  Species  and  "  Strains  "  of  Bacteria. — The 
original  "  genera  "  of  bacteria,  based  on  form  names 
such  as  bacillus  (rod),  coccus  (sphere),  spirillum  (spirally 
curved  rod),  are  still  to  some  extent  retained,  but  have 
been  considerably  added  to  as  knowledge  has  increased, 
and  the  present  classification  is  partly  based  on  form 
and  partly  on  activity.  Thus  rod  forms  are  still  called 
Bacterium  and  Bacillus,  the  genera  of  cocci  are  dis- 
tinguished by  prefixes  (Micrococcus,  Streptococcus — 
cells  arranged  in  chains,  Staphylococcus — cells  arranged 
like  bunches  of  grapes,  etc.).  Many  of  the  simpler 
spiral  forms  are  placed  in  Vibrio  and  Spirillum,  while 
the  long  spiral  forms  which  move  by  contractility  of 
the  body  protoplasm  and  have  no  flagella  are  placed 
in  Spirochcete.  Other  genera  are  Sarcina  (a  coccus 


SPECIES   AND   "  STRAINS  "  I4Q 

form  growing  in  rectangular  masses),  Nitrobacter  (oxi- 
dising nitrates  in  soil),  etc. 

The  species  of  bacteria  are  often  called  after  the  name 
of  the  discoverer,  or  from  their  form,  colour,  activity 
or  habitat :  in  the  case  of  the  pathogenic  (disease 
producing)  forms,  the  species  is  often  called  after  the 
name  of  the  disease,  e.g.  Bacillus  tuberculosis,  the  form 
causing  phthisis  and  other  forms  of  tuberculosis.  Some 
species  are  so  closely  alike  in  form  that  they  can  only 
be  distinguished  by  their  effect  upon  the  medium  in 
which  they  are  cultivated,  or  upon  the  animal  in  which 
they  live.  Thus  Bacillus  typhosus  of  typhoid  fever 
resembles  so  closely  B.  coli  communis,  a  regular  inhabi- 
tant of  the  human  intestine,  that  they  can  only  be 
distinguished  by  the  wide  difference  in  their  actions 
(for  instance  whether  they  can  or  cannot  ferment 
various  kinds  of  sugars),  the  former  producing  a  dan- 
gerous disease  through  the  substances  it  excretes. 

So  far  as  we  know  the  species  of  bacteria  are  as 
constant  as  those  of  other  organisms,  like  always  pro- 
ducing like,  but  different  "  strains  "  belonging  to  the 
same  species  may  differ  rather  widely  in  their  activities  ; 
for  instance  one  may  be  specially  virulent  as  a  disease 
producer,  while  another  may  give  rise  to  quite  mild 
symptoms,  so  that,  as  in  the  higher  plants,  we  have 
what  are  called  "  aggregate  species  "  and  "  elementary 
species "  (pure  strains).  How  far  strains  of  widely 
different  activities  are  produced  solely  by  the  effect 
of  changed  conditions,  we  do  not  certainly  know. 
Changed  conditions  undoubtedly  do  produce  great 
differences  ;  thus  by  growing  Bacillus  coli  in  a  culture 
medium  containing  a  trace  of  carbolic  acid  it  is  easy 
to  obtain  a  race  which  is  much  more  resistant  to  the 
disinfectant  action  of  the  phenol  than  is  the  normal 


I5O  BACTERIA 

species.  Among  the  enormous  number  of  kinds  of 
bacteria,  a  few  have  become  parasitic  on  the  higher 
animals  and  cause  disease.  Hence,  as  in  the  past, 
new  diseases  may  make  their  appearance  when  suit- 
able circumstances  occur,  by  the  adaptation  of  a  pre- 
viously harmless  form  to  new  conditions  in  an  animal 
body,  where  it  produces  poisonous  substances,  giving 
rise  to  the  symptoms  of  the  disease. 

Respiration  and  Fermentation. — Many  bacteria,  like 
the  vast  majority  of  animals  and  plants,  can  only 
live  in  the  presence  of  free  oxygen.  These  are  called 
aerobic  l  forms  (cf.  p.  132).  Some,  on  the  other  hand, 
like  yeast,  can  do  without  free  oxygen  for  a  time,  and 
others,  again,  cannot  live  in  its  presence  (anaerobic 
forms),  and  obtain  their  energy  by  splitting  up  organic 
substances  without  oxidation. 

Bacteria  which  live  on  complex  organic  substances, 
as  most  of  them  do,  nearly  always  cause  profound 
changes  in  the  medium  they  inhabit.  They  break 
down  the  organic  substances  of  the  medium  (pre- 
sumably by  the  excretion  of  enzymes)  just  as  yeast 
breaks  down  sugar.  The  term  fermentation  is  technically 
applied  to  all  such  processes,  and  is  not  confined  to 
alcoholic  fermentation  as  in  popular  usage.  Different 
kinds  of  bacteria  ferment  different  substances.  Thus 
certain  bacteria  found  in  milk  (Bacterium  acidi  lactici 
and  many  others)  ferment  milk  sugar  (lactose),  splitting 
one  molecule  of  lactose  (with  a  molecule  of  water) 
into  four  molecules  of  lactic  acid  : — 

C12H22On  +  H20  =  4CsH608 

lactose  water        lactic  acid 

and  the  accumulation  of  the  lactic  acid  in  the  milk 
turns  it  sour.  Alcohol  itself  is  split  up  by  certain 

1  Living  in  air,  from  Greek  ar)p,  air,  and  jStog,  life. 


VARIOUS   FERMENTATIONS  151 

bacteria,  especially  the  "  vinegar  plant,"  Bacterium 
aceticum,  in  the  presence  of  oxygen,  into  acetic  acid 
and  water :  — 

C2H6O  +  O2  =  C2H4O2  +  H2O 

ethyl  alcohol  acetic  acid 

Their  activity  is  the  cause  of  the  souring  of  wine  or 
beer  if  left  standing  in  the  air,  and  is  used  in  the  making 
of  vinegar.  The  "  ripening  "  of  cheeses,  the  "  curing  " 
of  tobacco,  the  "  retting  "  of  flax  (the  isolation  of  the 
fibres  of  the  flax  plant  when  the  stems  are  placed  in 
water),  and  many  similar  processes  useful  to  man 
depend  on  fermentations  carried  out  by  different 
bacteria ;  and  so  do  many  others  which  are  not  so 
welcome,  such  as  the  turning  rancid  of  butter  and  the 
turning  bitter  of  sweet  fruit  such  as  a  cut  melon,  as  well 
as  the  putrefying  of  meat,  etc. 

Putrefaction  and  Subsequent  Changes  in  Nitrogenous 
Organic  Substances. — Putrefaction  is  the  general  name 
given  to  various  fermentations  of  the  highly  complex 
proteins  which  form  the  basis  of  protoplasm.  These 
fermentations  result  in  the  formation  of  a  great  number 
of  compounds  of  varying  degrees  of  complexity  contain- 
ing nitrogen  and  sulphur,  among  which  are  the  evil- 
smelling  gases  so  characteristic  of  the  putrefactive 
processes.  Putrefaction  is  carried  out  by  various 
bacteria  which  are  very  widely  distributed  and  many  of 
which  form  spores,  so  that  practically  no  dead  body 
on  the  earth's  surface  can  escape  them.  The  process 
takes  place  in  all  dead  substances  rich  in  proteins  which 
remain  moist  and  not  too  hot  or  too  cold  to  allow 
of  the  activity  of  the  putrefactive  bacteria.  Freezing 
arrests  their  action,  and  so  do  the  antiseptic  substances 
formed  in  peat  bogs.  That  is  why  meat  is  kept  in  cold 


152  BACTERIA 

storage,  and  the  bodies  of  animals  which  have  been 
engulfed  in  bogs  are  often  found  perfectly  preserved 
after  many  years. 

After  the  first  putrefactive  decompositions  other 
kinds  of  bacteria  reduce  the  products  of  putrefaction 
to  simpler  substances  and  so  on  until  ammonium 
(NH4)  compounds  are  produced,  together  with  carbon 
dioxide  and  water.  This  process  is  noticed  in  the 
strong  smell  of  ammonia  (NH3)  arising  from  a  heap  of 
stable  manure  at  a  certain  stage  of  decomposition. 
The  debris  of  plants — fallen  leaves,  twigs,  etc. — undergo 
similar  decompositions,  but  since  a  smaller  proportion 
of  their  substance  is  formed  of  proteins  and  there  is 
a  much  larger  proportion  of  carbohydrates  (cellulose, 
etc.),  the  result  is  more  carbon  dioxide  and  water  and 
less  ammonia  than  in  the  case  of  animal  substances. 
Also  the  early  stages  of  decomposition  of  plant  remains 
are  largely  carried  out  by  fungi,  though  bacteria  also 
play  a  part.  The  decaying  plant  debris,  together 
with  any  decaying  animal  bodies  or  other  animal  sub- 
stances present,  form  the  humus  of  the  soil.  Humus  is 
always  in  process  of  reduction  to  the  simple  chemical 
substances  named  above,  together  with  various  mineral 
salts,  and  is  as  constantly  replenished  by  fresh  organic 
debris. 

When  the  organic  substances  have  been  brought 
into  the  state  of  simple  salts  containing  nitrogen, 
other  kinds  of  bacteria  take  up  the  work,  some  (Nitro- 
somonas)  converting  the  ammonium  salts  into  nitrites, 
i.e.  salts  formed  from  nitrous  acid  (HNO2)  and  bases 
present  in  the  soil,  others  again  (Nitrobacter)  carrying 
out  a  further  oxidation  and  producing  nitrates,  formed 
from  bases  and  nitric  acid  (HNO3).  This  process  of 
the  oxidation  of  ammonium  compounds  to  nitrates, 


CIRCULATION   OF   NITROGEN  153 

which  takes  place  in  two  stages,  each  effected  by  a  special 
set  of  bacteria,  is  called  nitrification. 

Nitrates  are  the  form  in  which  green  plants  mostly 
absorb  their  nitrogen  supply  through  their  roots,  so 
that  the  whole  series  of  bacterial  activities  described 
results  in  providing  green  plants  with  exactly  the  kind 
of  food  they  require,  and  at  the  same  time  gradually 
remove  dead  bodies  and  organic  debris  from  the  surface 
of  the  earth  and  make  room  for  fresh  life. 

During  the  progress  of  conversion  of  complex  nitro- 
genous compounds  into  simple  ones  a  great  deal  of 
nitrogen  is  lost  to  the  air  in  the  form  of  the  free  gas, 
but  certain  soil  bacteria  (e.g.  Azotobacter,  Clostridium) 
are  able  to  fix  this  and  incorporate  it  into  the  substance 
of  their  bodies.  Other  bacteria  (Pseudomonas)  which 
live  in  tubercles  that  are  formed  on  the  roots  of  certain 
plants  such  as  those  of  the  pea  family  (Leguminosce)  are 
also  able  to  absorb  free  nitrogen  from  the  air,  and  the 
green  plant  in  which  the  bacterium  lives  gets  the  benefit 
of  this,  eventually  breaking  up  and  absorbing  the 
products  of  the  dead  bodies  of  these  tubercle  bacteria. 
Thus  a  certain  amount  of  the  nitrogen  lost  to  the  air 
during  the  process  of  decomposition  is  again  brought 
into  the  form  of  living  protoplasm  in  these  ways. 

Circulation  of  Nitrogen  and  Carbon  in  Nature. — In 
all  these  ways  the  nitrogen  contained  in  the  proteins 
of  the  protoplasm  and  other  nitrogenous  substances  of 
plants  and  animals  is  converted  during  their  decay 
into  a  form  in  which  it  can  be  used  again  by  green 
plants,  so  that  a  constant  circulation  of  nitrogen  is  always 
going  on  in  nature,  through  the  agency  on  the  one  hand 
of  various  kinds  of  bacteria  which  break  down  the 
complex  substances  into  simple  ones,  and  on  the 
other  of  green  plants  which  build  them  up  again  into 


154  BACTERIA 

complex  ones.  In  this  circulation  the  animals  play 
a  comparatively  small  part,  for  they  merely  convert 
the  plant  proteins  into  the  proteins  of  their  own  bodies. 

Besides  the  circulation  of  nitrogen  there  is  also  a 
circulation  of  carbon,  which  appears  in  the  form  of 
carbon  dioxide,  not  only  as  the  result  of  the  respiration 
of  plants  and  animals,  but  also  during  decomposition 
of  the  complex  carbon  compounds  of  animal  and  plant 
bodies,  notably  the  carbohydrates  found  in  the  cellulose 
of  plants,  but  also  the  fats  and  proteins.  Distinct  bac- 
teria carry  out  the  disintegration  of  the  cellulose  of 
which  humus  is  largely  composed,  and  this  process 
ultimately  results  in  the  production  of  carbon  dioxide 
and  water  (as  well  as  hydrogen  and  marsh  gas).  The 
carbon  dioxide  is  then,  as  we  know,  used  by  the  green 
plant  to  build  up  its  body. 

Pathogenic  (Disease-producing)  Bacteria.— A  small 
proportion  of  the  total  number  of  species  are  partly 
or  wholly  parasitic  on  plants  and  animals  in  which 
they  cause  disease.  Partial  parasites  (such  as  Bacillus 
tetani)  live  naturally  in  soil  containing  organic  matter 
and  only  occasionally  gain  entrance  to  the  animal 
body.  Complete  parasites  (such  as  the  spirochaete  of 
syphilis,  or  the  coccus  of  cerebro-spinal  fever)  have  no 
life  outside  the  body,  and  their  existence  depends  on  their 
being  passed  on  directly  from  one  person  to  another. 

Bacteria  often  invade  living  animal  bodies  by  way 
of  wounds,  and  may  be  a  great  danger  to  life,  for  in- 
stance on  battlefields,  owing  to  the  poisons  or  toxins 
which  they  produce.  The  bacteria  are  carried  into 
the  wound  either  by  the  instrument  causing  the  wound, 
or  from  the  clothes,  or  by  dirt  afterwards  getting  on 
to  the  wounded  surface.  The  most  important  are  the 
species  of  Streptococcus  and  Staphylococcus  ;  and  these 


DISEASE   BACTERIA  155 

may  get  into  the  blood  stream  through  the  wound 
and  thus  be  carried  through  the  body,  bringing  about 
septiccemia  (blood  poisoning). 

The  immense  progress  of  modern  surgery  has  been 
made  possible,  first  by  the  use  of  antiseptics,  i.e.  by  treat- 
ing the  wound  with  a  substance  (such  as  weak  carbolic 
acid)  which  destroys  the  bacteria.  The  introduction 
of  the  use  of  these  in  surgery  was  due  to  Joseph  Lister 
(afterwards  Lord  Lister),  and  resulted  in  a  great  decrease 
in  the  death-rate  of  surgical  patients  from  blood  poison- 
ing. Later  on  aseptic  methods  of  surgery  were  intro- 
duced, in  which  the  surgeon's  hands,  instruments  and 
dressings  used  are  carefully  sterilised,  and  the  wound 
thus  kept  free  from  all  bacteria. 

Besides  the  Streptococci  and  Staphylococci,  other 
kinds  of  bacteria  may  be  introduced  into  wounds  con- 
taminated with  soil  rich  in  dung,  and  cause  such  specific 
diseases  as  tetanus  (Bacillus  tetani)  and  gas-gangrene 
(Bacillus  welchii),  so  called  because  of  the  large  amount 
of  gas  produced  in  the  wound  by  the  action  of  the 
bacillus  on  the  tissues.  These  are  both  spore-producing 
bacilli  which  inhabit  the  intestines  of  horses  and  cows, 
where  they  do  no  harm.  The  spores  persist  for  a  long 
time  in  the  manured  soil,  and  germinate  when  they 
enter  the  wound,  producing  active  cells  which  form 
dangerous  toxins  that  often  lead  to  death. 

In  the  case  of  most  "  infectious  "  diseases,  human 
beings  directly  or  indirectly  infect  one  another  with 
spores  or  active  cells  of  various  pathogenic  species, 
and  these  give  rise  in  the  body  to  different  specific 
toxins  which  cause  the  symptoms  of  the  corresponding 
disease.  Some  of  the  most  serious  among  them  are 
tuberculosis  in  its  various  forms  (Bacillus  tuberculosis), 
syphilis  (Spirochcete  pallida],  typhoid  or  enteric  fever 


156  BACTERIA 

(Bacillus  typhosus),  epidemic  meningitis  or  "  spotted 
fever "  (Meningococcus),  plague,  cholera,  diphtheria, 
influenza,  etc. 

PRACTICAL   WORK. 
BACTERIA. 

Note  that  all  bacteria  are  extremely  small,  and  very  careful 
focussing  with  the  high  power  is  necessary  to  find  them. 

(1)  Bacteria  in  Potato  Extract. — Mount  a  drop  of  the  turbid 
liquid  from  the  surface  of  water  in  which  a  slice  of  potato  has 
been  soaking,   put  on  a  coverslip  and  observe  the  numerous 
rod-shaped  aerobic  bacteria  (Bacillus  mesentericus)  in  motion  ; 
also  from  the  deeper  layer  of  water  the  rod  shaped  cells,  tapering 
at  the  ends  of  the  anaerobic  Clostridium  butyricum.     Note  the 
different  staining  of  the  two  with  iodine. 

(2)  Note  the  elongated  form  and  the  rapid  spiral  movements 
(like  those  of  eels)  of  the  Spirilla  from  horse  dung  which  has  been 
kept  in  water  for  a  week.     Kill  with  iodine  solution  and  again 
observe. 

(3)  Bacillus  subtilis  (hay  bacillus). — (a)  Mount  a  drop  of  hot 
water   infusion    of    hay,  and   observe   the   delicate  rod-shaped 
bacilli  in  motion,     (b)  Examine  a  preparation  showing  the  same 
species  fixed  and  stained.      Compare  their  size  with  that  of  a 
yeast  cell  by  means  of  the  micrometer  eyepiece. 

(4)  Stain  a  drop  of  each  of  the  bacterial  cultures  (2)  and  (30) 
by  the  coverslip  method  used  for  yeast  (p.  136  (4)). 

(5)  Examine  the  sections  of  a  nodule  of  the  root  of  lupin  (or 
other   leguminous    plant),    and   note    the    "  bacteroids "    (dead 
bacteria)  in  the  central  tissue. 

(6)  Remove  the  cover  from  the  dish  of  sterilised  gelatine  or 
agar  jelly  provided,  thus  allowing  the  dust  from  the  air  to  settle  on 
it.     Replace  the  cover  after  twenty  minutes,  and  label  the  dish 
with  your  name.     The  dishes  should  be  examined  again  after 
a  week's  interval. 

Demonstration  Specimens. — Examine  carefully  the  demonstra- 
tion slides,  tubes  and  plates  of  various  bacteria. 

[A  few  representative  types  of  culture  and  stained  preparations 
of  various  pathogenic  bacteria  should  be  obtained  for  exhibition. 
They  can  usually  be  borrowed  from  a  pathological  laboratory. 
Cultures  and  slides  of  Bacillus  tetani  and  B.  anthracis,  showing 
spores  of  B.  tuberculosis,  and  of  Staphylococcus  and  Streptococcus 
are  suggested  as  suitable.] 

The  student  should  be  warned  not  to  handle  the  cultures. 


CHAPTER   X 
SAPROPHYTIC  FUNGI 

MUCOR    AND    PENICILLIUM. 

THE  Fungi  differ  from  the  Bacteria  in  being  plants 
whose  bodies  are  composed,  not  of  single  cells  and 
cell  colonies,  but  of  delicate  branching  threads  which 
grow  in  liquid  or  solid  "  substrata  "  containing  organic 
substances  which  they  can  absorb  as  food,  and  often 
sending  branches  into  the  air.  This  system  of  branch- 
ing threads  is  called  the  mycelium,  and  the  individual 
branches  are  called  hyphce.  The  mycelium  of  many 
species  is  large  and  conspicuous.  As  a  general  rule 
fungi  can  only  grow  actively  in  damp  situations,  and 
they  are  commonest  when  and  where  the  air  contains 
a  large  amount  of  water  vapour. 

Fungi  may  be  either  saprophytic  or  parasitic  (see 
p.  78),  i.e.  they  may  either  take  their  food  exclusively 
from  the  tissues  of  a  living  organism  (plant  or  animal) 
or  they  may  take  it  exclusively  from  a  non-living 
organic  substance,  e.g.  the  dead  bodies  of  organisms, 
dung,  milk,  cheese,  jam,  leather,  etc.,  or  from  humus 
(seep.  154).  But  this  distinction  is  not  absolute.  For 
instance  a  parasitic  fungus  may  begin  its  life  in  the 
living  body  of  an  animal  or  plant  and  continue  to 
grow  after  the  "  host  "  is  dead.  Or  a  fungus  which 
is  ordinarily  a  saprophyte  may,  on  occasion,  attack 


I5§  SAPROPHYTIC   FUNGI 

the  living  body  of  an  organism.  A  fungus  which  is 
strictly  confined  to  parasitism  is  called  an  obligate 
parasite,  one  which  can  live  either  saprophytically  or 
parasitically  is  a  facultative  parasite. 

Moulds. — This  name  is  commonly  given  to  saprophytic 
fungi  which  grow  on  organic  substances,  provided  they 
contain  enough  water,  such  as  bread,  jam,  cheese, 
leather,  tobacco,  etc.,  on  which  they  produce  whitish, 
yellowish,  greenish  or  bluish  mycelia.  They  flourish 
particularly  when  the  air  is  very  damp  and  warm. 
In  damp  tropical  climates  any  such  substance  will 
become  covered  with  mould  in  a  day,  if  it  is  exposed 
to  the  air. 

Mucor. — One  of  the  commonest  genera  of  white 
moulds  is  Mucor,  which  forms  a  white,  fluffy  feltwork, 
rather  like  cotton  wool  in  appearance,  in  and  on  the 
surface  of  such  substances  as  those  mentioned.  If  a 
piece  of  this  feltwork  is  examined  under  the  micro- 
scope it  is  seen  to  consist  of  delicate  branched  threads, 
forming  the  mycelium  or  body  of  the  fungus.  The 
mycelium  of  Mucor  is  a  branched  non-cellular  tube, 
the  wall  enclosing  the  cytoplasm,  which  contains 
numerous  very  minute  nuclei,  though  these  cannot  be 
seen  except  by  special  methods  of  staining.  The 
cytoplasm  encloses  many  vacuoles,  which  are  smaller 
towards  the  growing  tips  of  the  hyphae.  The  actual 
tip  of  each  hypha  is  rounded  and  the  protoplasm  at 
the  apex  is  without  vacuoles.  As  the  hypha  grows 
in  length  the  nuclei  at  the  tip  constantly  divide. 

The  mycelium  branches  profusely,  many  of  the 
branches  penetrating  the  substratum  on  which  the 
mould  is  growing,  and  absorbing  liquid  food.  In  this 
way  it  acts  like  the  root  of  a  higher  plant,  except  that 
here  the  food  is  largely  organic.  The  essential  ele- 


VEGETATIVE   GROWTH  I5Q 

ment  carbon  is  taken  by  the  fungus  mainly  in  the 
form  of  a  soluble  carbohydrate  such  as  sugar. 
Nitrogen,  sulphur  and  the  other  necessary  elements 
are  absorbed  in  the  form  of  various  salts,  together 
with  water,  although  nitrogen  is  sometimes  absorbed  in 
the  form  of  very  complex  compounds.  The  mycelium 
also  branches  in  the  air  above  the  substratum,  but 
its  aerial  growth  is  necessarily  strictly  dependent  on 
the  extent  of  the  growth,  and  therefore  of  the  absorb- 
ing surface,  in  the  substratum. 

Here  for  the  first  time  among  the  types  we  have 
studied  we  meet  with  the  fixed  and  branching  habit 
characteristic  of  plants.  The  "  food  "  of  Mucor  exists 
all  round  it  in  the  bread  or  jam  or  other  organic  sub- 
stances which  forms  its  substratum,  and  the  branch- 
ing habit  of  its  "  root  "  puts  it  in  touch  with  as  much 
food  as  possible. 

Oxygen  for  respiration  is  obtained  from  the  air, 
and  the  necessity  of  free  oxygen  is  illustrated  by  the 
fact  that  the  mould  forms  a  cushion  only  on  and  in 
the  top  layer  of  jam  in  the  jam-pot.  It  is  probable 
that  the  mycelium  cannot  normally  penetrate  into  the 
deeper  layers  of  jam  because  of  the  deficiency  of  free 
oxygen  away  from  the  air.  Various  species  can, 
however,  like  yeast,  live  anaerobically  for  a  time  and 
can  produce  alcohol  and  carbon  dioxide  by  splitting 
sugar  molecules  without  oxidation. 

So  long  as  the  fungus  can  get  food,  water  and 
oxygen  it  continues  to  grow,  i.e.  the  nuclei  divide 
and  the  protoplasm  increases  in  amount  at  the  tips 
of  the  hyphae.  The  wall  covering  the  tip  is  con- 
tinuously pushed  out  and  new  wall  substance  is  con- 
tinuously added  to  make  good  the  thinning,  just  as 
in  the  growth  in  size  of  the  embryonic  cell  of  the 


160  SAPROPHYTIC   FUNGI 

higher  plant  (p.  107)  Every  now  and  then  the  grow- 
ing hypha  branches,  i.e.  a  new  branch  of  the  tube  is 
pushed  out  not  far  from  the  tip,  and  a  new  growing 
hypha  is  thus  produced.  This  continuous  indefinite 
growth  and  branching  of  the  plant  body  is  very 
characteristic  of  typical  plants  as  opposed  to  the 
limtied  compact  growth  of  the  typical  animal  (see 
Chapter  I,  p.  22).  From  what  is  known  in  the  case  of 
other  fungi  the  growing  tips  of  the  hyphae  are 
probably  sensitive  to  the  stimuli  of  food  substances 
and  of  free  oxygen,  i.e.  they  will  turn  and  grow 
towards  regions  whence  these  substances  are  diffusing 
towards  the  hypha. 

Reproduction. — (i)  Spore  formation.  In  place  of 
the  simple  cell  division  ol  Amoeba,  Protococcus  and 
bacteria,  and  the  simple  budding  of  yeast,  the  main 
method  by  which  Mucor  multiplies  and  produces 
new  individuals  is  the  formation  of  spores,  produced 
in  special  chambers  (sporangia)  cut  off  from  the  general 
mycelium.  Special  hyphae  (sporangiophores)  arise  as 
branches  of  the  mycelium  and  typically  grow  straight 
up  into  the  air.  These  sporangiophores  are  positively 
phototropic,  i.e.  they  grow  towards  brighter  illumi- 
nation. This  generally  involves  growing  away  from 
the  substratum,  so  that  the  sporangium  formed  at  the 
end  of  the  sporangiophore  is  placed  in  the  best  position 
for  free  distribution  of  its  spores  into  the  air. 

The  sporangium  itself  is  formed  by  the  swelling  up 
of  the  tip  of  the  hypha  (Fig.  16,  a),  so  that  under  a 
hand  lens  the  sporangiophore  looks  like  a  minute  pin 
with  a  globular  head  (the  sporangium).  A  cross  wall 
is  formed,  cutting  off  the  head  from  the  sporangiophore, 
and  the  protoplasm  of  the  sporangium  breaks  up  into 
a  number  of  tiny  multinucleate  cells,  each  of  which 


SPORE    FORMATION  l6l 

secretes  a  wall  upon  its  surface.  Directly  the  proto- 
plasm of  the  sporangium  breaks  up,  its  turgidity,  which 
has  been  maintained  by  the  osmotic  pressure  in  the 
vacuoles  against  the  semi-permeable  cytoplasm  sur- 
rounding them,  is  destroyed,  and  owing  to  the  osmotic 
pressure  being  still  maintained  in  the  sporangiophore 
on  the  other  side  of  the  cross  wall,  this  is  pushed  into 
the  cavity  of  the  sporangium  and  pressed  against  and 
into  the  mass  of  spores  (Fig.  16,  b).  This  pressure 
squeezes  the  spores  against  the  wall  of  the  sporangium  ; 
and  eventually,  with  the  progressive  drying  of  the 
wall  by  evaporation  (since  it  is  no  longer  in  contact 
with  the  protoplasmic  water),  the  wall  becomes  brittle 
and  is  burst,  the  spores  being  scattered  into  the 
air  as  a  fine  dust  (Fig.  16,  c). 

The  spores  float  in  the  air  and  are  easily  carried 
about  by  slight  currents.  The  air  of  rooms  and  of 
towns  generally  contains  innumerable  spores  of  Mucor 
and  other  moulds,  as  well  as  bacterial  cells  and  spores, 
and  these  gradually  settle  on  all  objects  when  the  air  is 
quiet.  Thus  any  suitable  organic  substance,  provided  it 
is  moist,  will  develop  moulds  or  colonies  of  bacteria, 
or  both,  because  the  spores  will  germinate  (or  the 
bacterial  cells  grow)  on  such  a  substance  from  which 
they  can  get  suitable  food.  The  protoplasm  of  the 
spore  absorbs  water  and  food,  the  wall  of  the  spore 
bursts,  and  the  protoplasm,  covered  by  a  new  delicate 
wall  (Fig.  16,  d),  is  pushed  out,  grows*  and  branches  to 
form  a  new  Mucor  plant. 

Only  the  air  of  regions  very  remote  from  human 
dwellings,  and  from  the  substances  on  which  moulds 
and  bacteria  thrive,  is  comparatively  free  from  spores, 
and  is  in  that  sense  pure.  The  purity  of  the  air  in 
this  sense  can  be  measured  by  the  number  of  mould 
II 


l62  SAPROPHYTIC   FUNGI 

and  bacterial  colonies  starting  on  a  given  surface  of 
sterilised  nutritive  agar  or  gelatine  which  has  been 
exposed  to  the  air  for  a  given  time. 

(2)  Chlamydospore  formation. — In    some    species    of 
Mucor  the  protoplasm  may  contract  into  oval  masses 
at  short  intervals  along  the  length  of  a  hypha   (for 
instance  a  sporangiophore)  the  short  diameter  of  the 
oval    corresponding    with    or    slightly    exceeding    the 
width  of  the  hypha.     Each  oval  mass  becomes  clothed 
with   an  independent   thick  wall,   which   often   bulges 
out   the   walls   of  the   parent   hypha.     The  chlamydo- 
spores    so    produced    become    free    by    the    breaking 
up  of  the   wall    of   the   now   empty   hypha,    and   are 
resistant  resting  spores  which    germinate    to    produce 
new    plants     under     favourable     conditions.      Similar 
chlamydospore    formation    is    not    uncommon    among 
fungi. 

(3)  Conjugation. — Quite    a    different    sort    of    repro- 
ductive process  sometimes  occurs  in  Mucor.     The  tips 
of  two  hyphse  arising  from  the  same  or  from  different 
branches  of  the  mycelium  approach  one  another,  and 
come  into  contact   (Fig.   16,  e),  each  swelling  into  the 
form  of  a  club.     A  cross  wall  is  formed  towards  the 
base  of  each  club-shaped  structure,  cutting  off  a  com- 
partment at  the  tip  (Fig.  16,  /).     One  of  these  is  fre- 
quently larger  and  more  swollen  than  the  other.     The 
walls    at    the    tips    which    are    in    contact    are    now 
absorbed    and    the    cytoplasm    of    the    two    compart- 
ments mingles.     The  nuclei  divide  actively  and  then 
conjugate  in  pairs.     The  walls  now  swell  up,  forming 
a  nearly  spherical  structure,  and  become  dark-coloured, 
often  black,  with  projecting  excrescences  on  the  sur- 
face (Fig.  16,  h).     Eventually  the   zygote   (product  of 
conjugation  so  formed)  separates  from  the  hyphae  on 


ZYGOTES   OF   MUCOR  163 

each  side.  The  zygotes  are  70  to  80  ju,  in  diameter 
and  just  visible  to  the  naked  eye.  Inside  the  dark, 
rough  outer  wall  are  two  layers  of  inner  wall. 

The  zygote,  in  the  protoplasm  of  which  a  consider- 
able amount  of  organic  food  material,  largely  fats,  is 
accumulated,  is  capable  of  remaining  quiescent  for  a 
considerable  time,  the  thick  outer  wall  resisting  desic- 
cation. In  a  suitable  medium  it  germinates,  the 
thick  outer  wall  bursting  and  the  thin  inner  wall  being 
pushed  out  by  the  protoplasm  and  growing  into  a 
new  mycelium  which  often  forms  a  single  sporangium 
at  once  (Fig.  16,  i). 

The  zygote  of  Mucor  is  a  resting  stage  in  the  life 
history,  preserving  the  plant  through  conditions 
unfavourable  to  active  growth  in  the  same  way  as 
the  spores  of  yeast  and  bacteria  ;  while  the  spores  of 
Mucor,  though  also  a  resting  stage  in  the  sense  that 
growth  of  their  protoplasm  is  temporarily  suspended, 
are  primarily  a  means  of  multiplying  and  dispersing 
the  individuals  of  the  species,  being  formed  in  im- 
mense numbers  and  much  less  resistant  than  the 
spores  of  bacteria. 

The  process  of  conjugation,  with  the  formation  of  a 
zygote,  or  product  of  conjugation,  in  Mucor  is  the 
first  example  of ,  this  process  we  have  met  with.  As 
we  shall  see  in  later  chapters,  this  process  is  almost 
universal  in  the  organic  world,  though  in  some  of  the 
lower  forms  it  is  apparently  absent.  In  Mucor  the 
two  conjugating  masses  of  protoplasm  are  often 
exactly  alike,  though  in  some  cases  one  hypha  is 
larger  than  the  other.  The  conjugation  of  two  equal 
protoplasts  is  called  isogamy.1 

In   some   kinds   of   Mucor   hyphae   from   the    same 

1  Greek  laos,  equal,  and  yajue'eo,  marry. 


164  SAPROPHYTIC   FUNGI 

mycelium  (even  from  the  same  branch)  will  conjugate, 
and  in  these  zygotes  are  commonly  found.  In  other 
kinds  zygotes  are  much  rarer,  since  conjugation  will 
only  occur  between  hyphae  of  two  different  "  strains," 
one  of  which  grows  more  vigorously  than  the  other. 
Here  we  have  a  functional  differentiation  between  the 
conjugating  hyphae  which  is  not  quite  on  a  par  with, 
but  shows  an  interesting  parallel  to,  the  ordinary  sex- 
differentiation,  the  essential  characters  of  which  we 
shall  have  to  consider  in  Chapter  XII. 

Penicillium  (Blue  Mould). — This  is  a  common  form 
growing  on  substances  similar  to  those  on  which 
Mucor  occurs.  It  forms  the  well-known  mould  of 
Stilton  and  Gorgonzola  cheese.  The  mycelium  differs 
from  that  of  Mucor  in  being  septate,  i.e.  there  are 
cross  walls  at  intervals,  dividing  the  mycelial  tube 
into  multinucleate  compartments.  The  characteristic 
blue-green  pigment  is  distributed  through  the  cyto- 
plasm, and  has  nothing  in  common  with  the  chlorophyll 
of  a  green  plant. 

Spore  Formation :  Conidia. — Upright,  aerial  hyphae 
arise  from  the  mycelium  as  in  Mucor,  but  instead  of 
forming  sporangia,  each  forms  several  branches  at 
the  same  level,  the  branches  continuing  the  same 
general  direction  of  growth  but  diverging  slightly 
from  one  another.  At  a  certain  height  each  of  these 
branches  again  in  the  same  way,  so  that  a  sort  of 
compound  pencil  of  hyphae  is  formed  with  the  tips 
of  their  ultimate  branchlets  close  together  on  the  same 
level.  From  the  tips  of  these  branchlets  minute 
spores  called  conidia  are  budded  off,  and  before  each 
is  quite  detached  another  is  budded  off  behind  it,  so 
that  a  series  of  parallel  or  nearly  parallel  chains  of 
conidia  are  produced  (Fig.  16,  /).  These  are  very 


PENICILLIUM  165 

lightly  connected,  so  that  a  slight  jar  or  a  current  of 
air  is  sufficient  to  detach  them,  and  they  float  off  into 


FIG.  16. — Reproduction  of  Mucor.  (i)  Spore  formation  (a-d)  : 
a,  young  sporangium  (sp) ;  b,  optical  section  of  mature  sporangium 
(col.,  columella)  ;  c,  remains  of  sporangium  wall  after  bursting, 
with  a  few  spores  still  adhering  ;  d,  germinating  spore  with  two  germ 
tubes.  (2)  e-h,  Conjugation  :  e,  conjugating  hyphae  in  contact ; 
/,  separation  of  tips  of  conjugating  hyphae  ;  g,  later  stage ;  h, 
mature  zygote  ;  t,  germination  of  zygote,  germ  tube  forming  a 
single  sporangium. 

Penicillium  :    j,    part   of   branched   conidiophore   budding   off 
chains  of  conidia. 


the  air,  where  they  behave  as  spores,  germinating  on 
any  favourable  substratum  on  which  they  may  happen 
to  settle. 


l66  SAPROPHYTIC   FUNGI 

There  is  also  a  complicated  method  of  conjugation 
which  is  comparatively  rarely  met  with  and  will  not 
be  described  here. 

Other  Saprophytic  Fungi. — There  are  numerous  other 
forms  of  saprophytic  fungi,  including  many  with  com- 
plicated reproductive  bodies  ("  fruit  bodies  "),  of  which 
the  common  mushroom  (Agaricus  campestris)  is  a 
good  example.  The  mycelium  lives  in  the  humus  of. 
the  soil,  particularly  in  well  manured  pastures,  and 
resembles  that  of  Mucor  in  a  very  general  way,  except 
that  it  is  septate,  as  are  all  the  higher  fungi.  The 
mushroom  itself  is  the  fruit  body,  which  arises  first  as 
a  small  weft  of  hyphae  forming  a  portion  of  the  fila- 
mentous mycelium.  This  increases  in  size  by  constant 
branching  and  grows  into  a  small  very  compact  nodule, 
which  increases  in  size  and  complexity,  becoming 
differentiated  into  a  short  stalk  and  an  arched  top. 
Finally  it  grows  up  above  the  soil  by  the  rapid  elonga- 
tion of  the  hyphae  forming  the  stalk  and  expands  the 
umbrella-shaped  pileus  J  or  top  of  the  mushroom.  From 
the  tips  of  the  hyphae  which  run  at  right  angles  to 
the  surfaces  of  the  platelike  "  gills  "  on  the  underside 
ol  the  pileus  there  are  budded  off  spores,  four  or  two 
from  each  hypha  tip,  each  formed  on  a  small  projecting 
point.  The  enormous  number  of  spores  produced  by  a 
single  mushroom  may  be  gauged  by  placing  a  ripe 
pileus,  detached  from  its  stalk,  underside  downwards 
on  a  piece  of  white  paper  and  leaving  it  for  a  day  or 
two.  On  gently  removing  the  pileus  a  pattern  of  the 
gills  will  be  found  designed  in  streaks  of  microscopic 
spores  on  the  paper. 

1  "  A  woven  hat." 


MUCOR   AND    PENICILLIUM  167 

PRACTICAL    WORK. 
MUCOR. 

(1)  Examine  with  a  hand  lens  the  young  mycelium  of  Mucor 
growing  in  gelatine  containing  some  nutritive  substance  such  as 
raisin-extract.     Cut  out  a   very  small   portion  of  the  gelatine 
containing  the  mycelium  near  its  edge,  and  place  it  in  a  large 
drop  of  water  on  a  slide  till  it  becomes  nearly  liquid.     Then 
put  on  a  coverslip  :    squeeze  it  gently  and  look  for  hyphae,  first 
with  the  low  and  then  with  the  high  power  of  the  microscope. 
Draw  a  portion  of  a  hypha,  including  its  tip,  under  the  high  power, 
showing  wall,  protoplasm  containing  vacuoles,  which  are  smaller 
towards  the  tip.     Note  the  absence  of  cross  walls. 

(2)  Examine  an  older  plant  (if  possible  growing  in  a  watchglass 
of  gelatine)   which   has  produced   sporangia — first  with  a  hand 
lens.     Then  place  the  watchglass  on  the  stage  of  the  microscope 
and  focus  slowly  down  with  the  low  power.     In  this  way  an 
excellent  idea  of  the  habit  of  growth  of  Mucor  is  obtained.     Note 
the  spherical  sporangia  of  different  ages  borne  on  the  summits 
of  the  sporangiophores,  and  the  branching  mycelium  below. 

Mount  some  of  the  mycelium,  carefully  removed  in  a  drop 
of  methylated  spirit  to  get  rid  of  air,  and  add  a  drop  of  dilute 
glycerine.  Find  and  draw  various  stages  in  the  development 
of  the  sporangia. 

(3)  Examine  a  still  older  plant  with  mature  sporangia.     Mount 
some  as  in  (2)  and  draw  stages  before  and  after  bursting.     Note 
the  columella.     Draw  also  a  few  spores. 

(4)  Examine  a  culture  or  a  prepared  slide  showing  zygotes. 
Look  for  and  draw  stages  in  conjugation  and  the  formation  of 
the  zygote. 

PENICILLIUM. 

(5)  Examine   Penicillium  (Blue   Mould)   in   the  same  way  as 
Mucor — first  in  situ  with  the  branched  conidiophores  rising  into 
the  air.     Examine  the  brushes  of  radiating  conidiophores  under 
the  low  power.     Examine  part  of  the  vegetative  mycelium  in 
gelatine  squeezed  out  in  water,  and  note  the  segmented  mycelium 
with  pale  blue-green  cytoplasm.     Draw  two  or  three  segments 
under  the  high  power. 

Carefully  remove  some  conidiophores,  dip  them  in  a  drop  of 
spirit,  and  mount  in  dilute  glycerine  ;  examine  under  the  high 
power  the  chains  of  conidia  and  their  mode  of  formation  by 
budding  from  the  tips  of  the  conidiophore  branches. 


l68  SAPROPHYTIC   FUNGI 

DEMONSTRATION  SPECIMENS. 

(6)  Examine  demonstration  specimens  of  representative  higher 
saprophytic  fungi,  such  as  the  Mushroom,  Puffball,  etc.  Note 
that  the  conspicuous  part  of  the  fungus  above  ground  is  a  compli- 
cated spore-bearing  fruit  body.  The  mycelium  lives  below  the 
surface  in  a  soil  rich  in  organic  matter. 

Examine  (if  available)  stages  showing  the  development  of 
the  mushroom  from  a  small  nodule  on  the  mycelium. 


CHAPTER    XI 

PARASITIC    FUNGI 

A  PARASITE,  as  we  have  seen  (p.  78)  is  an  organism 
which  gets  its  food  directly  from  another  living 
organism.  Bacteria  living  in  the  intestines  are  scarcely 
parasitic  in  the  strict  sense,  though  they  actually  live 
within  the  body  of  another  organism.  Many  of  them 
are  harmless  saprophytes  living  on  the  partly  digested 
food  of  the  animal,  but  some  cause  diseases  because 
they  produce  toxins.  Other  bacteria,  however,  live  in 
the  tissues,  which  they  break  down  by  means  of  the 
enzymes  they  secrete,  absorbing  some  of  the  products  : 
these  are  parasites  in  the  strictest  sense. 

Similarly  with  the  fungi.  Many  are  saprophytes  : 
some  of  these,  however,  are  able  to  attack  and  break 
down  the  tissues  of  living  organisms  (facultative  para- 
sites] ;  others  are  exclusively  parasites,  and  some  are 
strictly  confined  to  one  species  of  host. 

Fungi  Parasitic  on  Animals.— Though  the  great 
majority  of  "  zymotic  "  (germ)  diseases  of  man  and 
the  higher  animals  are  caused  by  bacteria  or  by  pro- 
tozoa (unicellular  animals),  e.g.  malaria,  sleeping  sick- 
ness, dysentery,  some — especially  skin  diseases — are 
caused  by  genuine  fungi  which  form  a  mycelium. 

The  skin  diseases  generally  known  as  ringworm  are 
among  the  commonest  of  these.  These  are  caused  by 
several  different  kinds  of  fungi.  Trichophyton,  whose 
mycelium  consists  of  chains  of  oval  or  rectangular 


170  PARASITIC   FUNGI 

cells  5  to  8  fi  in  diameter,  lives  in  and  feeds  upon  the 
layers  of  living  skin  cells.  These  living  skin  cells  are 
not  on  the  actual  skin  surface,  which  consists  of  dead 
cells  constantly  produced  by  division  of  the  living 
cells  below  the  surface  and  rubbed  off.  The  growth  of 
the  fungus  often  causes  local  inflammation,  and  the 
raising  of  the  skin  in  circular  pustules  corresponding 
with  the  centres  of  infection.  The  mycelium  grows 
outward  from  these  centres,  the  central  parts  dying 
off  and  thus  producing  the  ring-shaped  structures 
characteristic  of  the  disease. 

Ring-shaped  structures  formed  in  this  way  are  pro- 
duced by  many  fungi,  both  parasites  and  saprophytes, 
for  instance  the  "  fairy  rings  "  often  seen  in  meadows, 
and  also  sometimes  by  the  higher  plants,  for  instance 
grasses,  which  similarly  spread  outwards  from  a 
centre  by  vegetative  growth,  the  central  parts  dying 
off.  The  cause  of  the  dying  off  of  the  older  central 
parts  of  the  plant  may  be  either  the  exhaustion  of  the 
food  supply,  or  the  production  of  substances  which 
act  as  a  poison  to  the  plant  itself,  in  the  same  way 
that  the  accumulation  of  alcohol  and  carbon  dioxide 
in  fermenting  wort  eventually  stops  the  growth  of  the 
yeast  plant. 

Another  form  of  ringworm  is  caused  by  Microsporon, 
which  attacks  the  living  bases  of  the  hairs  of  the  skin, 
in  which  it  spreads,  forming  a  mycelium  of  rectangular 
cells,  and  causing  the  hair  to  break  off  short.  The 
mycelium  then  breaks  out  to  the  surface  of  the  hair, 
where  it  forms  abundant  spores  (2  fj,  in  diameter)  by 
thickening  of  its  cell  walls.  These  spores  are  easily 
brushed  off  and  render  the  disease  extremely  con- 
tagious. A  number  of  allied  species  live  on  dogs, 
cats,  horses,  etc.,  and  these  are  contagious  to 


FUNGAL  PARASITES   ON   ANIMALS  1?1 

man,  generally  infecting  the  hairs  of  the  scalp  in 
children. 

Other  fungal  diseases  of  the  skin  or  mucous  mem- 
branes are  pityriasis,  due  to  Microsporon  furfur,  which 
forms  a  spore-producing  mycelium  on  the  skin, 
especially  in  phthisical  patients  ;  and  "  thrush,"  caused 
by  a  fungus  which  has  a  septate  mycelium  forming 
white  patches  on  the  tongue  and  on  the  mucous  mem- 
brance  of  the  throat,  mostly  in  children,  and  also 
producing  spores,  round  or  oval  in  shape. 

"  Blastomycosis  "  of  the  lung  or  kidney  is  caused 
by  single  cells,  which  form  endogenous  spores,  like 
yeast,  and  may  also  produce  a  mycelium  in  artificial 
cultures.  This  kind  of  fungus  is  also  found  on  the 
skin,  often  following  a  slight  wound,  and  causing 
suppuration  (pus  formation).  Another  fungus  which 
inhabits  the  lung  cavities  is  Aspergillus  fumigatus, 
which  forms  a  spore-bearing  mycelium  and  spreads 
into  the  bronchioles  (small  branching  air  tubes  of  the 
lungs).  This  form  is  generally,  but  not  always,  found 
in  company  with  disease-producing  bacteria.  Asper- 
gillus also  occurs  in  the  external  ear. 

Saprolegnia  and  Fish  Disease.— A  good  example  of  a 
facultative  parasite  is  Saprolegnia,  which  lives  sapro- 
phytically  on  the  dead  bodies  or  parts  of  animals  and 
plants  that  have  fallen  into  or  died  in  the  water,  but 
which  also  attacks  living  aquatic  animals,  such  as 
insects,  fish  and  amphibia.  A  spore  germinating  on 
the  body  of  an  aquatic  insect,  for  instance,  sends  out 
a  tube  which  penetrates  the  tissues  of  the  insect,  and 
branches  to  form  a  mycelium  which  ramifies  through- 
out the  body.  This,  of  course,  causes  the  death  of 
the  insect,  and  the  fungus  lives  saprophytically  for  a 
time  on  the  dead  tissues.  Eventually  thick  branches 


172  PARASITIC   FUNGI 

of  the  mycelium  are  put  out  into  the  water,  and  each 
is  cut  off  by  a  cross  wall,  forming  a  sporangium.  The 
protoplasm  of  the  sporangium  then  divides  to  form  a 
number  of  spores.  These  are  motile  spores  called 
zoospores,1  oval  in  shape  and  with  flagella  attached  to 
the  pointed  end.  They  are  set  free  by  the  bursting  of 
the  sporangium  wall,  swim  about  in  the  water,  and 
germinate,  producing  a  new  mycelium  if  they  find  a 
suitable  organic  substratum,  living  or  dead.  Sexual 
organs  are  also  formed  on  the  mycelium.  These  will 
not  be  described  here,  but  it  may  be  noted  that,  as 
is  often  the  case  in  fungi  which  form  sexual  organs, 
conjugation  may  not  take  place,  the  female  cell  (egg) 
developing  into  a  new  plant  without  fertilisation. 
This  behaviour  is  called  parthenogenesis. 

Saprolegnia  sometimes  attacks  fish,  particularly  carp 
and  gold-fish  in  stagnant  ponds,  entering  by  the  gills, 
which  it  often  blocks  up,  causing  the  death  of  the 
fish.  According  to  some  authorities  it  is  a  species  of 
Saprolegnia  which  causes  salmon  disease,  attacking 
the  gills  of  the  salmon,  and  sometimes  killing  them  in 
great  numbers,  thus  causing  extensive  damage  to 
salmon  fisheries.  Other  authorities,  however,  believe 
that  bacteria  are  the  primary  cause  of  this  disease  in  the 
salmon,  which  are  attacked  by  Saprolegnia  only  when 
the  fish  are  already  weakened  by  the  bacterial  attack. 

Fungal  Parasites  of  Plants. — Fungi  parasitic  on  the 
higher  plants  are  far  more  numerous  in  species  and  far 
commoner  than  those  which  attack  animals.  They  often 
cause  widespread  and  serious  damage  to  crop  plants. 

Pythium  de  Baryanum  (causing  the  "  damping  off  " 
of  seedlings). — Closely  sown  young  seedlings  in  a 
moist  atmosphere,  such  as  that  of  a  greenhouse,  some- 

1  Greek  £a>ov,  animal,  from  the  motility  of  the  spores. 


"  DAMPING-OFF  "    OF   SEEDLINGS  173 

times  topple  over  owing  to  the  dying  of  the  stem 
just  where  it  emerges  from  the  soil  (Fig.  17,  G).  This 
is  called  "  damping  off,"  and  is  due  to  the  attack  of 
a  fungus  (Pythium)  which  has  formed  an  extensive 
branching  mycelium  in  the  tissues  of  the  seedling. 
The  mycelium  itself  ramifies  in  the  air  spaces  (inter- 
cellular spaces)  between  the  living  cells  of  the  host 
(Fig.  17,  A,  B),  but  it  pierces  the  living  cells  where  it 
touches  them  (Fig.  17,  B),  breaking  down  their  proto- 
plasm. The  seedling  is  soon  killed  and  the  fungus 
lives  on  for  a  time  in  the  decaying  stem.  Eventually 
the  ends  of  some  of  the  hyphae  swell  up  to  form  a 
conidium  (Fig.  17,  A,  sp.,  and  C),  which  is  cut  off  from 
the  hypha  by  a  cross  wall,  and  becomes  detached. 
In  damp  air  this  may  grow  out  at  once  to  form  a  new 
mycelium  which  penetrates  a  fresh  seedling,  but  in 
water  it  puts  out  a  short  tube  at  the  end  of  which 
a  similar  body  is  formed  (Fig.  17,  D).  This  is  a 
zoosporangium  (cf.  Saprolegnia)  whose  protoplasm 
divides  to  form  zoospores  (z),  which  on  being  set  free 
by  the  bursting  of  the  sporangium  wall  swim  about 
in  the  water.  The  zoospores  germinate  (zj  to  form 
new  mycelia  which  penetrate  fresh  seedlings.  Zoospores 
are  never  produced  except  in  water. 

Sexual  organs  are  also  produced.  The  female  organ 
is  a  spherical  structure  formed  by  the  swelling  of  the 
end  of  a  hypha  which  is  cut  off  from  the  rest  by  a 
cross  wall.  In  this  spherical  structure  the  protoplasm 
contracts  to  form  a  smaller  sphere  (egg).  The  male 
organ  is  club-shaped,  and  is  also  cut  off  from  the 
hypha  by  a  cross  wall  (Fig.  17,  E). 

The  male  organ  applies  itself  closely  to  the  female 
organ  (a),  and  a  short  tube  from  the  former  penetrates 
the  latter  (6).  Through  this  the  male  protoplasm 


PARASITIC   FUNGI 


FIG.  17, — Pythium.  A,  hyphae  in  tissue  of  seedling;  sp.,  young 
sporangium.  B,  ditto,  more  highly  magnified.  The  hyphae  (h) 
have  penetrated  the  cell  walls  (c.w.)  through  narrow  holes  which 
the  growing  tips  have  bored  out.  C,  branched  hypha  emerged 
into  the  air,  forming  sporangia  (sp.}.  D,  germination  of 
sporangium  in  water.  The  contents  have  divided  to  form  a 
number  of  zoospores  which  are  enclosed  in  a  bladder  pushed 
through  a  hole  in  the  wall ;  z,  free  zoospore ;  zt  germinating 
zoospore.  E,  sexual  reproduction  (conjugation)  ;  a,  young 
female  organ  (spherical)  and  male  organ  (club-shaped)  in  contact ; 
b,  penetration  of  male  organ  ;  c,  fertilised  egg  (zygote)  surrounded 
by  thick  wall,  male  organ  empty.  F,  germination  of  zygote 
in  water  (similar  to  D).  G,  cress  seedling  attacked  by  Pythium 
just  above  the  soil  surface.  (After  Marshall  Ward,  etc.) 


POTATO   BLIGHT  175 

passes  to  coalesce  with  that  of  the  female.  The  fertilised 
egg  (zygote)  now  secretes  a  wall  (c),  and  remains  for  a 
time  in  a  resting  condition.  Eventually  it  germinates, 
forming  zoospores  in  water  (F),  which  germinate,  and 
if  the  hyphae  of  this  find  a  suitable  seedling  they  pene- 
trate it  through  a  stoma  just  above  the  ground  level. 

The  hyphae  of  Pythium  can  only  grow  in  air  which 
is  nearly  saturated  with  water  vapour,  but  these  con- 
ditions are  often  realised  between  the  stems  of  the 
seedlings  growing  in  a  seed-box  which  is  frequently 
watered.  When  a  seedling  has  been  killed  the  hyphae 
grow  out  from  it  and  easily  bridge  the  space  to  another 
seedling,  which  they  enter  and  kill,  and  so  on  till  the 
whole  crop  quickly  becomes  a  rotting  mass.  The 
hyphae  of  the  mycelium  are  easily  visible  as  very 
delicate  white  filaments,  rather  like  cobweb,  stretching 
between  the  seedlings. 

Potato  Blight  (Phytophthora  infestans). — This  is  a 
fungus  closely  allied  to  Pythium,  and  often  causes 
enormous  damage  to  the  potato  crop.  Potato  blight 
first  appeared  in  Europe  between  1845  and  1850  and 
ruined  the  Irish  crop,  causing  serious  and  widespread 
famine.  Since  then  it  has  always  been  with  us,  but 
its  incidence  is  much  more  severe  in  some  years  than 
in  others,  according  to  the  weather. 

The  mycelium  ramifies  in  the  tissues  of  the  leaf, 
and  the  diseased  patches  turn  brown  as  the  tissues 
die.  Under  conditions  favourable  for  the  spread  of 
the  disease  the  whole  potato  haulm  is  soon  reduced  to  a 
rotting  mass.  If  the  under  side  of  a  potato  leaf  bear- 
ing a  patch  of  the  disease  is  examined  with  a  hand 
lens,  the  edge  of  the  patch  is  seen  to  be  covered  with 
very  delicate  white  glistening  threads  (the  hyphae) 
which  have  grown  out  through  the  stomata  of  the 


176  PARASITIC   FUNGI 

leaf  (Fig.  18,  F).  Minute  white  bodies  can  sometimes 
be  seen  on  the  threads.  These  are  the  conidia,  like 
those  of  Pythium,  cut  off  from  the  ends  of  the  hyphae 
(Fig.  18,  E,  F,  A).  They  are  formed  in  great  abundance 
in  damp  air  and  are  easily  carried  by  the  wind  from 
one  plant  to  another.  In  water,  for  instance  in  a 
raindrop  or  dewdrop  on  a  leaf,  the  contents  of  the 
conidium  break  up  into  zoospores  (Fig.  18,  B,  C),  as 
in  Pythium,  and  each  of  these  eventually  germinates 
to  form  a  hypha  (D),  which  infects  the  leaf  by 
growing  in  through  a  stoma  or  by  piercing  the  cuticle. 
The  conidia  may  also  be  carried  down  by  rain  into 
the  soil,  where  they  infect  the  young  potato  tubers 
which  are  growing  beneath  the  surface.  Whether 
the  fungus  can  survive  the  winter  in  the  soil  and  rein- 
feet  the  seed  tubers  in  the  spring  is  not  certainly 
known.  One  means  of  transmission  of  the  disease 
from  season  to  season  is  by  mycelium  carried  over  in 
certain  of  the  tubers  used  for  "  seed."  Such  tubers 
are  not  sufficiently  diseased  to  be  detected,  but  in 
the  early  summer  some  of  them  may  give  rise  to 
diseased  shoots  on  which  air-borne  conidia  are  formed. 
These  act  as  centres  of  infection  to  surrounding  plants. 
Phytophthora  appears  on  the  leaves  of  the  potato 
plant  in  the  damp  south-west,  for  instance  in  southern 
Ireland,  Cornwall  and  the  Isle  of  Wight,  as  early  as 
the  end  of  May.  From  these  regions  it  develops  suc- 
cessively north-eastwards  across  the  country.  It  usually 
appears  in  Cambridgeshire  and  Lincolnshire  for  instance 
— great  potato-growing  counties — about  the  end  of 
July  or  the  beginning  of  August,  but  its  spread  and 
the  severity  of  the  disease  depend  very  largely  on 
the  weather.  A  warm,  nearly  saturated  atmosphere, 
with  constant  south-westerly  winds,  are  the  conditions 


POTATO   BLIGHT 


177 


most  favourable  to  the  spread  of  the  fungus,  for  in 
such  conditions  the  conidia  are  formed,  scattered  by 
the  wind,  and  germinate,  with  the  greatest  rapidity. 


FIG.  18. — Conidia  and  zoospores  of  Potato  Blight  (Phytophthora 
infestans).  A,  conidium  cut  off  from  the  end  of  a  hypha. 
X  500.  B,  conidium  evacuating  zoospores.  x  5°°-  C.  two 
zoospores.  X  500.  D,  zoospore  germinating.  E,  branched 
conidiophore.  The  conidia  have  fallen  from  the  tips  of  three 
branches,  one  is  shown  free  and  one  still  attached.  The  branch 
on  the  right  is  just  forming  two  conidia.  X  120.  F,  young 
conidiophore  protruding  through  a  stoma  and  bearing  a  young 
conidium  at  its  tip.  x  200.  (After  Frank.) 

12 


178  PARASITIC   FUNGI 

In  a  dry  summer  the  effects  of  the  disease  are  usually 
negligible. 

Potato  blight  cannot  be  cured  once  it  has  got  a 
hold  on  the  plant,  but  its  spread  can  be  very  largely 
checked  by  spraying  the  leaves  with  "  Bordeaux 
mixture."  This  is  a  "  solution  "  of  copper  sulphate 
and  lime  ;  it  dries  on  the  leaves,  and  when  wetted 
again  by  rain  or  dew  it  forms  a  poisonous  copper 
solution  which  kills  the  zoospores,  germ  tubes  and 
young  hyphae.  In  this  way  the  crop  can  be  very 
efficiently  protected  against  the  onset  of  the  disease. 
Even  if  the  disease  is  only  moderately  severe  spraying 
will  increase,  or  more  strictly  will  prevent  a  diminution 
of,  the  potato  crop.  By  destroying  the  zoospores  and 
young  hyphae  it  enables  the  leaves  to  go  on  making  sugar 
and  proteins,  essential  for  the  growth  and  stocking  with 
starch  of  the  potato  tubers,  during  August  for  instance, 
at  a  time  when  they  might  otherwise  be  injured  and 
their  chlorophyll  largely  destroyed  by  the  fungus,  even 
if  the  shoot  of  the  plant  were  not  entirely  killed, 
j  Rust  of  Wheat  (Puccinia  graminis  and  P.  glumarum). 
— Puccinia  graminis  is  an  example  of  a  highly  speci- 
alised parasite  with  a  much  more  complicated  life 
history  than  that  of  Pythium  or  of  Phytophthora.  On 
the  leaf  or  stem  of  the  wheat  plant  straw-coloured  or 
reddish  streaks  may  appear  in  June  or  July.  These 
are  masses  of  uredospores,  a  special  kind  of  conidia 
(Fig.  19,  A),  which  are  formed  on  the  ends  of  hyphse  that 
have  burst  through  the  surface  of  the  leaf  or  stem 
from  a  mycelium  in  the  tissues  below.  Detached  and 
blown  about  by  the  wind,  the  uredospores  germinate 

FIG.  19. — Life  history  of  Rust  Fungus  (Puccinia).  A,  isolated 
uredospores  of  P.  graminis  (black  rust).  X  475.  B,  germination 
or  uredospore  showing  penetration  of  the  germ  tube  through  a 
stoma  into  the  wheat  leaf.  Note  the  futile  attempts  at  branching 


LIFE   HISTORY   OF   RUST   FUNGI 


179 


B 


on  the  surface,  and  the  aggregation  of  protoplasm  in  the  branched 
apex,  x  475.  C,  single  teleutospore.  X  200.  D,  germination 
of  teleutospore  of  another  rust  fungus  and  formation  of  sporidia. 

X  475.  E,  aecidium  cup  of  another  form  in  vertical  section 
(cross-section  of  leaf)  ;  myc.,  mycelium  of  the  fungus ;  ce,  aecidio- 
spores;  It. ,  leaf  tissue;  ep.t  epidermis  of  leaf.  (After  De  Bary.) 

X  150.  F,  germination  of  aecidiospore  on  a  grass  leaf,  penetration 
of  a  stoma  by  and  branching  of  the  germ  tube.  X  475.  (All  except 
E  after  Plowright.) 


l8o  PARASITIC   FUNGI 

in  a  drop  of  rain  or  dew  on  the  surfaces  of  fresh  wheat 
leaves,  and  the  germ  hyphae  enter  the  stomata 
(Fig.  19,  B),  branch  within  the  leaf,  and  penetrate  the 
living  cells.  Hyphae  from  this  mycelium  burst  out 
from  the  surface  of  the  leaf  again,  forming  fresh  masses 
of  uredospores  ;  and  so  the  process  is  repeated,  and 
the  fungus  spreads  during  the  latter  part  of  the  grow- 
ing season  from  one  wheat  field  to  another. 

The  uredospores  formed  about  the  time  the  crop  is 
harvested  can  survive  the  winter  and  are  capable  of 
infecting  the  young  wheat  plants  in  the  following 
spring.  But  another  kind  of  spore,  the  teleutospore,1 
two-celled  and  with  thick  dark  walls  (Fig.  19,  C),  are 
also  formed  in  numbers  towards  the  end  of  the  season 
by  the  mycelium  of  Puccinia  graminis  (the  Black 
Rust).  The  teleutospore  hibernates,  and  in  the  spring, 
instead  of  germinating  directly  to  form  a  new  mycelium, 
sends  out  short  thin  hyphae  which  cut  off  small  thin 
walled  conidia  called  sporidia  (Fig.  19,  D),  and  these  do 
not  infect  the  wheat  plant,  but  instead  the  leaves  of 
the  barberry  (Berberis  vulgar  is),  a  not  uncommon 
shrub  of  hedgerows.  The  germ  tubes  from  the  sporidia 
form  mycelia  within  the  barberry  leaf,  and  these  form 
spores  of  a  fourth  kind  (cecidiospores)  in  cup-shaped 
structures  (acidia)  on  the  surface  of  the  leaf  (Fig.  19,  E). 
At  the  base  of  each  cup-shaped  structure  there  is  a 
mass  of  parallel  hyphae  at  right  angles  to  the  surface 
of  the  leaf,  and  from  the  end  of  each  a  chain  of 
aecidiospores  is  budded  off  in  much  the  same  way  that 
the  conidia  of  Penicillium  are  budded  off  from  the 
end  of  the  conidiophores  (p.  164).  These  aecidiospores 
are  detached,  and  if  they  germinate  on  a  wheat  leaf 

1  Greek  TeAetrn},  end,  because  they  are  formed  at  the  end  of  the 
season  of  active  growth  of  the  fungus. 


PLANT   DISEASES  l8l 

the  germ  tube  penetrates  a  stoma  (Fig.  19,  F)  and  in- 
fects the  wheat  plant  again,  producing  a  mycelium  which 
forms  uredospores,  the  type  with  which  we  started. 

Thus  the  fungus  may  live  on  alternate  hosts  (cf.  the 
bacilli  of  tetanus  and  gas-gangrene,  p.  155),  the  wheat 
and  the  barberry  ;  though  it  may  also,  by  wintering 
in  the  uredospore  condition,  live  exclusively  on  the 
wheat  plant.  Long  before  this  complicated  life  his- 
tory was  worked  out,  farmers  had  noticed  that  if 
there  were  barberry  bushes  in  their  hedges  their  wheat 
was  particularly  liable  to  "  rust,"  and  in  some  countries 
it  is  a  legal  offence  to  allow  the  barberry  on  a  farm. 
Wheat  rust  often  causes  losses  to  the  wheat  crop  of 
the  world  represented  by  millions  sterling. 

Puccinia  graminis  (the  Black  Rust)  is  rare  on 
wheat  in  this  country.  Puccinia  glumarum  (the  Yellow 
Rust)  is  much  commoner,  but,  so  far  as  is  known, 
only  two  spore  stages  occur  in  the  life-history  of  this 
species — uredo-  and  teleuto-spores. 

There  are  thousands  of  different  kinds  of  parasitic 
fungi  known  which  infest  wild  and  cultivated  plants, 
some  living  exclusively  on  one  species  of  host,  some, 
like  black  rust  of  wheat,  living  on  alternate  hosts,  and 
others  being  less  exclusive  in  their  habits.  They 
often  do  severe  and  widespread  damage  to  important 
crops  and  cause  heavy  losses  in  money.  In  the 
seventies  of  the  last  century,  for  instance,  the  coffee- 
planting  industry  of  Ceylon  was  literally  destroyed 
by  a  fungus  (Hemileia  vastatrix)  parasitic  on  the  coffee 
plant.  The  study  of  the  life  histories  and  habits  of 
these  parasitic  fungi  forms  one  of  the  principal  branches 
of  plant  pathology,  which  has  become  within  recent 
years  a  study  of  great  practical  importance.  For  it 
is  only  by  a  careful  and  thorough  study  of  the  life 


l82  PARASITIC   FUNGI 

histories  and  necessary  conditions  of  life  of  the  different 
fungal  parasites  that  we  can  get  the  knowledge  neces- 
sary to  enable  us  to  devise  methods  of  preventing  or 
mitigating  their  attacks.  The  cure  of  plant  diseases 
caused  by  fungi  is  seldom  possible  ;  but  preventive 
measures,  taken  after  a  thorough  knowledge  has  been 
obtained  of  the  parasite  and  of  the  conditions  under 
which  the  disease  spreads,  are  often  very  successful. 
Just  as  in  the  case  of  animal  diseases,  "  prevention  is 
better  than  cure,"  but  with  plants  it  is  almost  the 
only  method  which  is  of  any  use  at  all. 

Bacteria  are  the  cause  of  some  plant  diseases,  just 
as  we  have  seen  that  mycelial  fungi  cause  certain 
animal  diseases.  But  the  latter  are  far  more  wide- 
spread as  pathogenic  plant  parasites,  just  as  bacteria 
are  by  far  the  most  widespread  and  destructive  of 
the  parasites  of  animals.  This  difference  is  mainly 
due  to  two  causes.  First  the  spores  of  fungi  are  pro- 
duced and  carried  about  mainly  in  air,  while  many 
bacteria  are  largely  carried  about  in  the  blood-stream 
which  is  absent  from  plants  ;  and  with  this  the  second 
cause  is  connected.  The  life  processes  of  fungi  are 
adjusted  to  ordinary  air  temperatures,  and  the  tempera- 
tures of  the  bodies  of  plants  exceed  these  by  very 
little.  On  the  other  hand,  the  life  processes  of  patho- 
genic bacteria  go  on  much  more  rapidly  at  higher 
temperatures,  such  as  that  which  is  maintained  in  the 
body  of  a  warm-blooded  animal  (about  37°  C.). 

PRACTICAL  WORK. 
"  WHITE  RUST  "  (Albugo). 

(i)  The  white  pustules  on  the  shoots  of  the  Shepherd's  Purse 
(Capsella)  are  caused  by  a  parasitic  fungus  (Albugo)  which  often 
deforms  the  inflorescence.  The  mycelium  of  the  fungus  sends 
out  hyphae  which  bud  off  chains  of  conidia  just  below  the  surface 
of  the  host  in  such  numbers  that  they  burst  off  the  surface 


PRACTICAL   WORK  183 

layer  of  tissue.  Scrape  off  a  small  portion  of  the  pustule, 
mount  in  a  drop  of  water  and  examine  with  the  high  power. 
Draw  some  of  the  spherical  conidia  formed  by  the  club-shaped 
hyphae  (conidiophores). 

(2)  In   cross-sections  through   a    pustule   (sections  should   be 
cut  in  alcohol  and  mounted  in  dilute  glycerine)  note  the  layer 
of  closely  packed  chains  of  conidia  perpendicular  to  the  surface, 
and  the  mycelium  in  the  tissue  of  the  host. 

(3)  In  sections  through  older  pustules  note  the  oogonia,  each 
containing  a  fertilised  egg  (zygote)  in  the  interior  of  the  tissue 
of  the  host. 

FUNGI  PARASITIC  ON  ANIMALS. 

(4)  From  the  dead   fly  or  ant's   "  egg  "    (pupa)   infested   by 
Saprolegnia  '  (which  attacks  both  living  and  dead  aquatic  animals) 
scrape  off  a  small  portion  of  the  surface,  mount  in  a  drop  of  water, 
and  cover.     Under  the  high  power  note  the  hyphae,   and  the 
long  club-shaped  sporangia  containing  spores  (which  afterwards 
escape  and  swim  in  the  water  as  zoospores,  settling  and  germinat- 
ing on  other  living  or  dead  aquatic  animals). 

(5)  If   available,  examine   demonstration   specimens  of  other 
parasitic  fungi  growing  in  the  bodies  of  fish  or  of  aerial  insects 
(flies,  caterpillars,  etc). 

(6)  Examine  slides  showing  the  spores  of  ringworm  (Microsporon) 
on  the  bases  of  human  hairs. 

"  TRUE  "  RUST  FUNGI. 

(7)  The  wheat  (or  other  grass)  leaf  provided  bears  orange  or 
reddish  pustules  produced  by  the  rust  fungus  Puccinia.     Examine 
first  with  a  hand  lens  and  then  place  the  leaf  dry  on  a  slide  and 
examine  with  the  low  power.     Note  that  the  surface  layer  of 
the  leaf  is  burst  along  the  line  of  the  pustule,  which  is  formed  of 
masses  of  spores   (uredospores).     Scrape  off  some  of  these  and 
examine  under  the  high  power  in  a  drop  of  water.     Draw  one  or 
two  spores  and  look  for  thin  places  in  the  wall  through  which 
germination  will  take  place. 

(8)  Examine    similarly    the    leaf-bearing     "  aecidium     cups  " 
formed  by  close-set  chains  of  spores  (aecidiospores)  cut  off  from 
ends  of  hyphae  in  the  leaf.     This  is  another  stage  of  a  rust  fungus 
(Puccinia) . 

(9)  Examine  any  fresh  or  museum  specimens  of  other  parasitic 
fungi  that  may  be  available. 

1  Easily  obtained  by  keeping  dead  flies  or  ants'  "  eggs  "  in  water 
brought  in  from  a  stagnant  pond  containing  much  organic  debris. 


CHAPTER   XII 

ORIGIN    OF    SEX    AND    OF    THE    SOMA.     THE 
GREEN   ALG.E 

LIFE  probably  began  in  the  sea,  and  some  at  least  of 
the  earliest  organisms  were  probably  minute  free- 
floating  forms  which  were  able  to  absorb  and  use 
light-energy  to  build  up  their  bodies  from  simple 
inorganic  substances,  and  must  therefore  be  classed  as 
plants.  Of  these  some  developed  flagella  and  became 
actively  free  swimming,  and  their  descendants  are 
still  represented  by  yellow,  brown  or  green  flagellate 
unicellular  plants  (algae)  which  live  in  the  sea  or  in 
fresh  water.  All  these  free-floating  and  free-swimming 
organisms,  whether  animals  or  plants,  are  collectively 
called  plankton.1  One  series  of  green  flagellate  plankton 
algae,  nearly  all  of  which  are  confined  to  freshwater, 
illustrates  very  beautifully  the  way  in  which  the 
differentiation  of  sex  among  conjugating  cells  came 
into  existence,  and  the  same  series  also  illustrates  the 
origin  of  what  is  called  the  soma*  that  is  the  body 
of  an  organism  as  opposed  to  its  reproductive  cells. 

"  Immortality  "  of  Unicellular  Organisms. — The  body 
of  a  unicellular  form,  such  as  an  amoeba,  a  bacterium 
or  a  yeast  plant,  not  only  feeds  and  grows,  it  also 
divides  (or  buds)  and  produces  new  individuals  of  the 
species.  This  production  of  new  individuals,  or  repro- 
duction as  it  is  called,  is  in  origin  simply  an  extension 

1  Greek  TrAay/cro'g,  wandering.  2  ati/ia,  body. 

184 


"  SOMA  "    AND    "  GERM   CELLS  "  185 

of  the  growth  process  :  it  is  discontinuous  growth,  as 
we  saw  in  Chapter  V,  because  it  is  growth  conditioned 
by  the  separation  of  a  part  or  parts  of  the  individual 
to  form  new  individuals.  The  protoplasm  of  the 
individual  organism  continues  to  exist  and  to  increase 
in  bulk,  but  it  can  only  increase  beyond  a  certain 
limit  of  size  if  it  separates  to  form  two  or  more  new 
individuals.  Under  favourable  conditions  of  life  this 
process  continues  indefinitely,  so  that  the  protoplasm 
of  which  the  organism  is  composed  is  immortal  in  the 
sense  that  it  need  never  die  so  long  as  the  conditions 
remain  favourable  to  its  continued  life,  though  it 
dies  of  course  as  soon  as  the  conditions  become  suffi- 
ciently unfavourable.  But  in  the  higher  organisms — 
in  the  great  majority  of  multicellular  animals  and 
plants — the  functions  of  nutrition  and  growth  are,  as 
we  know  very  well,  separated  from  the  function  of 
reproduction.  The  feeding  and  growing  body  does 
in  most  cases  regularly  die  whether  the  general  condi- 
tions of  life  continue  favourable  or  not.  It  dies,  as 
we  say,  of  old  age  :  it  is  a  soma  or  "  mortal  "  body. 
On  the  other  hand,  the  reproductive  cells  (germ  cells] 
which  it  produces  grow,  under  certain  conditions,  into 
new  individuals  of  the  species.  The  organisms  we  are 
now  going  to  consider  will  help  us  to  understand  how 
this  separation  of  functions  came  to  arise. 

Chlamydomonas. — The  organisms  belonging  to  this 
genus  are  very  common  in  pools,  rain-water  tanks, 
etc.  Each  is  a  minute  green  cell  (Fig.  20,  a),  oval  or 
sometimes  rather  oblong  in  shape,  with  a  basin-shaped 
chloroplast  occupying  the  hinder  end  of  the  cell,  con- 
taining usually  one  conspicuous  pyrenoid,  and  enclosing 
central  colourless  cytoplasm  containing  the  spherical 
nucleus.  The  front  end  of  the  cell  also  consists  of 


i86 


ORIGIN   OF   SEX   AND   OF   THE   SOMA 


colourless  cytoplasm  to  which  are  attached  two  long 
delicate  flagella  that  protrude  into  the  water  in  which 
the  organism  lives  :  by  the  rhythmical  lashing  of  the 
flagella  Chlamydomonas  swims  actively  about,  con- 
tinually rotating  on  its  long  axis  as  it  moves  forward, 
like  a  rifle  bullet  in  flight.  The  protoplasm  of  the 
cell  is  closely  covered  with  a  cellulose  cell  wall  which 
thins  away  at  the  front  end  to  which  the  flagella  are 
attached.  A  red  eyespot  on  the  surface  of  the  proto- 
plasm, containing  carotin  (see  p.  112),  which  renders 


FIG.  20. — Chlamydomonas:  a,  diagram  of  swimming  individual 
(vegetative  cell)  ;  b-d,  stages  of  division  to  form  four  new 
swimming  individuals;  c.w.,  cell  wall;  chl.,  chloroplast;  pyr., 
pyrenoid;  c.c.,  central  cytoplasm;  n,  nucleus;  c.v.,  contractile 
vacuoles  ;  St.,  eyespot ;  fl,  flagella. 

the  organism  sensitive  to  the  direction  of  light,  and 
two  very  small  contractile  vacuoles  just  below  the 
insertion  of  the  flagella,  complete  the  equipment  of 
the  cell. 

The  nutrition  of  Chlamydomonas  resembles  that  of 
Protococcus  (p.  73).  It  can  live  and  flourish  in  a 
weak  solution  of  the  proper  inorganic  salts  (such  as 
that  given  on  p.  123),  which  provide  it  with  the  neces- 
sary elements  for  building  up  its  protoplasm  ;  and  it 
makes  sugar  and  starch  (laid  down  round  the  pyre- 


REPRODUCTION  OF   CHLAMYDOMONAS  187 

noid)  from  the  elements  of  carbon  dioxide  and  water. 
It  is  in  fact  in  its  manner  of  feeding  essentially  a 
green  plant,  its  constant  active  movement  notwith- 
standing. We  have  every  reason  to  believe  that  the 
main  series  of  green  plants  are  derived  from  similar 
motile  forms. 

Reproduction  of  Chlamydomonas.  —  (a)  Vegetative 
Division. — In  this,  the  commonest  process  of  repro- 
duction, the  cell  comes  to  rest,  the  protoplasm  with- 
draws from  the  wall,  the  nucleus  divides  into  two, 
and  this  process  is  followed  by  the  division  of  the 
cytoplasm,  including  the  chloroplast,  a  furrow  appear- 
ing on  the  surface  and  extending  inwards  till  separation 
is  complete  (Fig.  20,  6).  Sometimes  the  process  of 
division  stops  there,  and  each  new  cell  produces 
flagella  and  secretes  a  cell  wall  on  its  surface.  The 
two  daughter  individuals  begin  to  move  actively,  by 
lashing  their  flagella,  within  the  cell  wall  of  the  mother, 
which  they  burst  and  then  swim  out  into  the  water, 
leaving  the  empty  shell  of  the  mother  cell  wall  behind. 
Each  grows  to  the  size  of  the  mother  individual.  We 
see  that  this  process  is  essentially  the  same  as  the 
division  of  an  amoeba  into  two  new  amoebae,  except 
that  here  the  dead  rigid  cell  wall  is  left  behind.  Under 
favourable  conditions  of  life  division  takes  place  once 
every  day,  towards  evening,  and  is  complete  in  a  few 
hours. 

Very  often,  however,  division  does  not  stop  with  the 
first  bipartition,  but  each  daughter  cell  divides  again 
(Fig.  20,  c),  so  that  four  daughter  individuals  are 
formed  instead  of  two  (Fig.  20,  d}.  These  of  course 
are  correspondingly  smaller,  but  on  escape  from  the 
mother  wall  they  quickly  grow  to  the  full  size. 

(b)  Formation  of   Gametes    and  Conjugation. — Some- 


i88 


ORIGIN  OF  SEX  AND  OF  THE   SOMA 


times  the  process  of  division  continues  further,  so 
that  by  repeated  bipartitions  from  eight  to  sixty-four 
cells  are  formed,  their  size  varying  inversely  as  the 
number  produced.  In  some  species  they  secrete  cell 
walls,  in  some  their  bodies  are  naked,  but  in  other 


FIG.  21. — Conjugation   of   Chlamydomonas  :   a,  gametes   approaching 
one  another ;    b  and  c,  stages  of  fusion ;  d,  zygote.     (After  Dill.) 

respects  they  are  identical  with  the  ordinary  indi- 
viduals. When  these  larger  numbers  of  small  daughter 
cells  are  produced,  they  do  not,  as  a  rule,  at  once 
grow  into  full-sized  individuals,  but  conjugate  in  pairs, 
and  are  hence  called  gametes.1  In  the  process  of  con- 

«  Greek  yct/i&o,  marry. 


CONJUGATION   IN   CHLAMYDOMONAS  189 

jugation  two  gametes  swim  towards  each  other 
(Fig.  21,  a),  their  front  ends  come  into  contact,  and 
their  bodies  gradually  fuse  completely  (Fig.  21,  b, 
Fig.  22,  a,  b),  cytoplasm  with  cytoplasm,  and  nucleus 
with  nucleus.  When  the  gametes  have  cell  walls  the 
protoplasmic  bodies  slip  out  of  them,  leaving  the 


FIG.  22. — Conjugation  of  Carteria  muttifilis,  a  species  with  four  flagella 
closely  allied  to  Chlamydomonas  :  a,  beginning  of  fusion  ;  b, 
advanced  stage  ;  c,  zygote  with  eight  flagella  ;  d,  zygote  with 
thick  wall  after  loss  of  flagella.  (After  Dill.) 

empty  shells  behind  (Fig.  21,  c).  The  single  cell  formed 
by  the  union  of  the  two  gametes  (Fig.  21,  d)  is  called 
the  zygote  (cf.  Mucor,  p.  162  ;  Pythium,  p.  175).  In  some 
cases  the  zygote  continues  to  swim  for  a  time  with 
its  four  flagella,  two  derived  from  each  gamete 
(Fig.  22,  c — this  is  drawn  from  a  form  in  which  the 


IQO  ORIGIN   OF   SEX   AND   OF  THE   SOMA 

single  cell  has  four  flagella  and  the  zygote  therefore 
eight),  but  eventually  it  loses  the  flagella,  becomes 
perfectly  spherical  and  secretes  a  thick  cell  wall 
(Fig.  22,  d),  going  into  the  resting  state,  in  which  con- 
dition it  can  resist  a  certain  amount  of  desiccation 
(cf.  Mucor,  Pythium,  etc.).  On  germination  the  proto- 
plasmic contents  of  the  zygote  divide  to  form  two  or 
four  ordinary  (vegetative)  individuals. 

(c)  Differentiation  of  Sex. — When  the  gametes  are  all 
equal  in  size,  as  they  are  in  several  species  of  Chlamy- 
domonas,  they  are  called  isogametes.1  But  in  certain 
species  they  are  of  two  sizes,  the  larger  derived  from 
fewer,  the  smaller  from  a  greater  number  of  divisions 
of  the  mother  cell.  Such  gametes  are  called  hetero- 
gametes.*  Conjugation  then,  so  far  as  has  been 
observed,  takes  place,  only  between  a  large  and  a 
small  gamete.  In  one  species  (C.  monadina),  in  which 
the  details  of  the  process  have  been  carefully  observed, 
and  in  which  all  the  gametes  have  cell  walls,  the 
anterior  ends  of  the  two  come  into  close  contact,  as 
in  the  case  of  isogametes,  and  the  cell  walls  of  the 
two  become  fused  together  (Fig.  23,  a).  The  proto- 
plasm of  the  small  gamete  separates  from  its  wall 
(sometimes  secreting  a  new  wall  round  its  hinder 
end,  Fig.  23,  B),  and  slips  through  the  channel 
formed  by  the  fusion  of  the  two  into  the  cavity  of 
the  large  gamete,  where  it  eventually  fuses  completely 
with  the  body  of  the  larger,  nucleus  with  nucleus  and 
cytoplasm  with  cytoplasm — the  chloroplasts  with  their 
pyrenoids  remaining  distinct  longest  (Fig.  23,  c). 
The  mass  of  protoplasm  (zygote)  so  formed  contracts, 
becomes  spherical,  and  secretes  a  thick  cell  wall  within 
the  cell  cavity  of  the  large  gamete  (Fig.  23,  c). 

1  Greek  loot;,  equal,  and  yc^ueco.  *  ereQoq,  other  (different). 


SEXUAL   DIFFERENTIATION  IQI 

Now,  here  we  have  two  leading  characters  of  the 
sexual  differentiation  of  gametes,  a  difference  in  size 
and  a  difference  in  activity.  Though  both  gametes 
begin  life  as  free  swimming  cells  of  identical  structure, 
one  is  more  active  than  the  other  in  the  actual  pro- 
cess of  conjugation.  This  difference  in  activity  seems 
here  to  be  a  mere  mechanical  result  of  the  difference 
in  size.  If  we  suppose  the  protoplasm  of  each  gamete 


FIG.  23. — Conjugation  of  Chlamydomonas  monadina.  a,  Beginning 
of  fusion  of  the  small  (male)  and  large  (female)  gamete.  (Note 
that  each  has  the  normal  structure  of  a  vegetative  Chlamy- 
domonas-cell.)  b,  Further  stage  of  fusion  (drawn  on  a  larger 
scale).  Note  the  central  area  of  colourless  cytoplasm,  derived 
from  the  front  ends  of  the  two  gametes,  and  now  containing 
the  two  nuclei  (equal  in  size)  in  contact  but  not  yet  fused. 
c.  Spherical  zygote  formed  within  the  cell  wall  of  the  female 
gamete  and  itself  clothed  with  a  cell  wall.  Note  that  the  two 
gamete  nuclei  have  now  fused  to  form  the  single  zygote  nucleus, 
but  the  chloroplasts  are  still  separate.  (After  Goroschankin.) 

to  be  attracted  equally  strongly  towards  the  other, 
the  body  of  the  small  one  would  be  more  easily  drawn 
through  the  comparatively  narrow  canal  formed  between 
them  ;  and  this  conclusion  is  supported  by  the  unusual 
cases  figured  in  Fig.  24.  Here  the  gametes  have  come 
into  contact  obliquely,  or  have  swung  round  after  con- 
tact, so  that  no  canal  is  formed,  but  the  protoplasm  of 
both  slips  out  of  its  cell  wall  and  the  two  form  a  zygote 


IQ2  ORIGIN   OF   SEX   AND   OF  THE   SOMA 

outside,  just  as  in  the  case  of  the  walled  isogametes 
in  Fig.  22. 

In  forms  which  show  a  more  complete  differentiation 
of  sex,  as  we  shall  presently  see,  not  only  the  size 
but  also  the  structure  of  the  two  conjugating  gametes 
is  very  markedly  different,  so  that  the  large  one 
(female)  is  necessarily  quite  passive,  and  the  small 
(male)  is  alone  active  in  the  process. 

We  now  have  to  consider  a  series  of  forms  which 
do  not  live  singly  like  Chlamydomonas,  but  in  which 
the  cells  produced  by  ordinary  division,  each  of  which 


a.  b. 

FIG.  24. — Unusual  type  of  conjugation  in  the  same  species.  The 
gametes  have  met  obliquely,  and  the  protoplasm  of  both  has 
emerged  from  the  cell  walls  to  form  a  spherical  zygote  outside. 

has  the  same  essential  structure  as  a  Chlamydomonas 
cell,  remain  together,  surrounded  by  a  common  (muci- 
laginous) envelope,  through  which  the  flagella  pro- 
trude. The  colony  or  ccenobium  :  of  cells  so  formed 
behaves  like  a  single  organism,  moving  through  the 
water  by  the  co-ordinated  beating  of  the  flagella  of 
all  the  cells.  The  first  form  we  shall  consider  is 

Pandorina,  which  is  a  spherical  ccenobium  of  (usually) 
1 6  cells  pressed  closely  together,  so  that  each  cell  is 
somewhat  wedge-shaped  (Fig.  25,  A).  In  reproduction 
each  cell  of  the  ccenobium  divides  by  4  closely  follow- 
ing bipartitions  to  produce  16  cells,  and  each  group 
of  16  cells  forms  a  new  ccenobium  (Fig.  25,  B),  the 

1  Greek  K0iv6s,  common,  and  fito$,  life. 


PANDORINA 


193 


FIG.  25. — Pandorina.  A,  colony  (ccenobium)  of  16  cells  in  the 
vegetative  state.  B.  reproductive  condition — each  cell  of  the 
coenobium  has  divided  to  form  a  new  daughter  coenobium.  C, 
free-swimming  gametes  of  various  sizes ;  a,  6,  conjugation  of 
gametes  of  equal  size  ;  c,  of  unequal  size  ;  d,  zygote  still  retaining 
the  flagella  of  the  gametes  which  formed  it ;  e,  conjugation 
between  small  (male)  gametes  which  have  originated  elsewhere 
and  large  sluggish  (female)  gametes  which  have  remained  where 
they  were  formed  by  division  ;  /,  walled  zygote  ;  g,  escape  of 
contents  of  zygote  as  a  free-swimming  zoospore  which  will  form 
a  new  coenobium  by  division. 

13 


IQ4  ORIGIN   OF  SEX  AND  OF  THE  SOMA 

16  young  ccenobia  escaping  from  the  mother  envelope 
into  the  water  and  then  growing  to  the  full  size.  The 
formation  of  gametes  takes  place  by  similar  divisions, 
but  this  proceeds  to  various  extents,  thus  producing 
gametes  of  different  sizes.  The  individual  gametes  so 
formed  become  loosened  from  the  mother  envelope, 
all  but  the  largest  escaping  singly  and  swimming  about 
individually  in  the  water  (Fig.  25,  C).  Gametes  are  pro- 
duced at  the  same  time  from  a  number  of  ccenobia 
lying  close  together,  so  that  the  thin  mucilage  becomes 
full  of  swimming  gametes  of  all  sizes.  These  con- 
jugate in  pairs  without  reference  to  size  (a,  b,  c),  except 
that  the  largest,  which  have  remained  where  they 
were  formed,  can  only  conjugate  with  the  small  active 
gametes,  which  seek  them  out  (Fig.  25,  e).  Spherical 
zygotes  are  formed  (d),  which  become  covered  with  a 
cell  wall  (/),  from  which  the  protoplasm  eventually 
escapes  (g)  as  a  flagellated  cell,  and  this  divides  to 
form  a  new  ccenobium. 

Here  then  we  have  an  even  earlier  stage  in  the 
evolution  of  sex  than  in  Chlamydomonas  monadina — 
an  intermediate  condition  between  isogamy  and  hetero- 
gamy — for  there  is  no  sharp  division  into  two  sizes, 
the  largest  gametes  alone,  which  do  not  move  from 
their  places,  representing  the  female  condition,  the 
smaller  either  acting  as  isogametes  (a)  or  as  males  (e). 

Eudorina  and  Pleodorina. — These  are  forms  with 
spherical  ccenobia,  larger  than  Pandorina,  and  with  all 
the  cells  (32  to  128)  spherical  and  separate,  forming 
a  single  layer  on  the  surface  of  the  ccenobium.  Each 
cell  is  of  the  Chlamydomonas  type  of  structure.  In 
Eudorina  all  the  cells  of  the  ccenobium  are  alike  and 
all  take  part  in  division  to  form  new  ccenobia,  which 
takes  place  just  as  in  Pandorina.  The  gametes  are, 


EUDORINA   AND    PLEODORINA  195 

however,  strongly  differentiated.  The  female  gametes 
are  very  much  like  the  ordinary  vegetative  cells  except 
that  they  soon  lose  their  flagella.  They  are  not  set 
free  from  the  colony  in  which  they  are  formed,  which 
scarcely  differs  in  appearance  from  a  vegetative  colony. 
The  male  gametes,  on  the  other  hand,  are  formed  by 
division  of  each  cell  of  a  vegetative  colony  into  64 
cells  (male  gametes)  forming  a  plate,  and  each  of 
these  is  long  and  narrow,  tapering  at  the  end,  the 
green  chloroplast  being  represented  only  by  a  yellow 
coloration  at  the  hinder  end.1  The  plate  of  male 
gametes  swarms  out  as  a  whole,  like  a  ccenobium, 
but  soon  breaks  up  into  its  constituent  gametes,  each 
of  which  is  capable  of  seeking  out  and  conjugating  with 
a  female  gamete,  the  process  resulting  in  the  forma- 
tion of  a  zygote.  Here  we  have  the  third  character 
of  sex  differentiation — a  difference  in  structure  between 
the  two  gametes,  as  well  as  the  difference  in  size  and 
activity  seen  in  Ghlamydomonas  monadina  and  to 
some  extent  in  Pandorina.  It  is  to  be  noted  that  the 
male  gamete  has  suffered  reduction  in  its  nutritive 
equipment,  i.e.  its  chloroplast,  as  well  as  in  size. 

In  Pleodorina  illinoiensis  (Fig.  26,  A)  the  coenobium  is 
almost  identical  with  that  of  Eudorina,  except  that 
(usually)  four  of  the  cells  at  the  front  end  of  the 
coenobium  (which  is  often  elliptical  in  shape)  are 
smaller  than  the  others.  When  division  occurs  to 
form  new  ccenobia  these  four  smaller  cells  do  not 
divide  like  the  others ;  and  they  remain  behind, 
eventually  dying,  when  the  daughter  ccenobia  escape. 
Here  we  have  the  first  indication,  in  this  series  of 
organisms,  of  the  appearance  of  a  soma  or  mortal 

1  The  male  gametes  of  Endorina  are  very  much  like  those  of  Volvox 
(see  p.  200  and  Fig.  28,^!)). 


ig6  ORIGIN   OF  SEX   AND   OF   THE   SOMA 

body  which  takes  no  part  in  reproduction.  It  is 
difficult  to  say  why  these  particular  cells  should  be 
incapable  of  division.  Perhaps  it  may  be  connected 
with  a  more  highly  developed  function  for  perceiving 
the  direction  of  light.  It  is  generally  true  that  speciali- 
sation of  vegetative  function  in  a  cell  carries  with 
it  loss  of  reproductive  power.  It  is  noteworthy  that 
all  transitions  are  found  between  the  Eudorina  condi- 
tion, in  which  all  the  cells  are  equal  and  capable  of 


B 


FIG.  26. — Pleodorina.  A,  P.  illinoiensis.  Coenobium  consisting  of 
28  large  cells  capable  of  division  to  form  new  coenobia  and  4  small 
cells  at  the  front  end  of  the  colony  which  do  not  divide  and  die 
when  the  new  ccenobia  formed  by  the  large  cells  are  liberated. 
B,  P.  californica.  Sterile  (somatic)  cells  more  numerous.  The 
arrow  indicates  the  direction  of  locomotion  in  both  cases. 

division,  and  the  occurrence  of  these  sterile  cells,  which 
may  vary  both  in  size  and  in  number  from  one  to 
twelve,  and  that  the  capacity  for  division  depends  upon 
the  size  of  the  cell.  Pleodorina  illinoiensis  is  in  fact  con- 
sidered to  be  a  state  of  Eudorina,  and  thus  we  have 
the  appearance  of  the  soma,  a  fundamentally  im- 
portant step  in  evolution,  first  arising  as  a  fluctuating 
condition  within  the  limits  of  a  species. 

A   larger   form    of    Pleodorina    (P.    californica)    has 


PLEODORINA   AND   VOLVOX  IQ7 

been  described  (Fig.  26,  B),  typically  consisting  of  128 
cells,  of  which  only  about  one-half,  situated  in  the 
hinder  part  of  the  ccenobium,  are  capable  of  division, 
while  the  remaining  cells  in  the  front  part  of  the 


FIG.  27.  Volvox  awrews.  x  180.  (After  Klein.)  A  coenobium  which  has 
produced  all  three  kinds  of  germ  cells.  The  three  large  (walled) 
spherical  cells  are  fertilised  eggs,  the  small  groups  of  cells  are 
derived  from  androgonidia,  which  will  divide  further  to  form 
sperms  (cf.  Fig.  28,  D),  the  largest  spheres  are  young  ccenobia 
derived  from  parthenogonidia ;  division  in  these  last  is  complete, 
but  the  cells  have  not  yet  separated.  Note  that  about  a  quarter 
of  the  mother  ccenobium  (the  front  quarter)  is  free  from  repro- 
ductive cells.  The  arrow  shows  direction  of  locomotion.  Compare 
Fig.  26,  A  and  B. 

coenobium  are  somatic,  i.e.  sterile  and  incapable  of 
division.  This  marks  a  further  step  in  the  restriction 
of  the  reproductive  (germ)  cells. 

Volvox. — In  Volvox,  the  largest  and  most  highly 
differentiated  member  of  this  series  of  forms,  the 
coenobium  consisting  of  many  hundreds  of  cells,  the 


IQ8  ORIGIN  OF  SEX  AND  OF  THE  SOMA 

development  of  the  soma  has  been  carried  much 
further.  Here  all  the  cells  of  the  ccenobium  are 
somatic  (Fig.  27),  i.e.  purely  vegetative  in  function  and 
incapable  of  division,  with  the  exception  of  a  limited 
number  of  purely  reproductive  or  germ  cells.  These 
germ  cells  are  of  three  kinds  :  (i)  so  called  partheno- 
gonidia  (Fig.  28,  A),  each  of  which  divides  to  form  a 
new  ccenobium  (Fig.  28,  B),  and  thus  corresponds  in  its 
reproductive  function  with  the  ordinary  vegetative  cells 
of  a  Pandorina  or  of  a  Eudorina ;  (2)  gynogonidia 
(Fig.  28,  C),  which  develop  directly  into  passive  (female) 
gametes  (eggs)  without  flagella,  these  eggs  being  very 
large  cells  highly  stored  with  food  ;  and  (3)  andro- 
gonidia,  each  of  which  divides,  as  in  the  case  of 
Eudorina,  already  described,  to  form  a  plate  (Fig.  28,  D) 
or  a  sphere  of  male  gametes  (sperms)  *  which  have  the 
same  general  characters  as  those  of  Eudorina.  These 
three  types  of  reproductive  cell  may  be  distributed 
in  any  combination.  Thus  a  ccenobium  may  reproduce 
entirely  by  means  of  parthenogonidia,  or  it  may  pro- 
duce eggs  only,  or  colonies  of  sperms  only,  or  any 
two,  or  all  three  forms  of  germ  cell  (Fig.  27). 

There  are  two  species  of  Volvox  found  in  Britain,  V. 
aureus  (Figs.  27,  28,  A-D)  and  V.  globator.  The  former 
has  rounded  somatic  cells  which  are  separated  from  one 
another  by  considerable  spaces,  which  are  bridged  bydeli- 

1  Greek  airepfia,  seed.  This  is  a  useful  term  applied  to  all  male 
gametes  both  in  animals  and  plants ;  the  male  fertilising  element  has 
often  been  called  the  "  seed  "  in  common  language. 

FIG.  28.  Volvox.  A,  portion  of  the  surface  of  a  coenobium  showing 
vegetative  cells  joined  to  one  another  by  from  one  to  three  proto- 
plasmic threads :  also  a  parthenogonidium  (non-sexual  germ 
cell),  x  550.  B,  partly  grown  daughter  ccenobium  derived 
from  the  division  of  a  parthenogonidium.  x  550.  C,  egg 
joined  to  neighbouring  vegetative  cells  by  bundles  of  protoplasmic 
threads,  x  about  700.  D,  isolated  "  plate "  of  male  gametes 
(sperms)  derived  by  division  of  an  androgonidium.  x  700. 


VOLVOX 


E,  isolated  sperms  of    V.  aureus  (note  chloroplast,  eyespot  and 
flagella).      x  825.      F,  smaller   sperms    of    V.   globator   on    same 
scale  (chloroplast  degenerate).     G,  egg  of    V.  globator  surrounded 
by  sperms,      x  400.     (After  Klein  and  Cohn.)     A-E,   V.  aureus. 

F.  G.   V.  globator. 


200  ORIGIN   OF   SEX   AND   OF   THE   SOMA 

cate  threads  of  cytoplasm  (Figs.  27,  28,  A-C).  V.  globalor, 
which  is  larger  and  less  variable  in  size,  has  its  cells 
much  closer  together  and  joined  to  one  another  by  stout 
angular  projections  from  the  cytoplasm  of  the  cell 
bodies  (Fig.  29).  It  is  interesting  to  note  that  V.  aureus 
has  larger  green  male  gametes  (Fig.  28,  E),  while 
V.  globator  has  smaller  very  thin  sperms,  with  the  hinder 
end  yellow  and  the  two  flagella  usually  attached  about 
the  middle  of  the  body  (Fig.  28,  F),  thus  departing 
much  further  than  the  sperms  of  V.  aureus  from  the 
structure  of  the  primitive  Chlamydomonadine  cell, 
though  still  showing  clear  traces  of  derivation  from 
that  structure.  The  sperms  of  V.  globator  are  nearly 


FIG.  29. — Vegetative  cells  of  V.  globator  seen  in  profile  of  surface  of 
coenobium.  Note  cell  walls  and  broad  protoplasmic  connexions. 
The  cell  on  the  left  is  a  young  parthenogonidium.  x  1,600. 

as  much  reduced  and  highly  specialised  as  those  of 
green  plants  very  much  higher  in  the  scale  of  vege- 
tative structure.  The  extreme  contrast  between  the 
male  and  female  gametes  in  size,  shape  and  structure 
is  very  obvious  (Fig.  28,  G),  though  each  may  be 
clearly  derived  from  the  Chlamydomonadine  cell 
through  the  stages  of  differentiation  we  have  traced. 

Nature  and  Significance  of  the  Differentiation  between 
Male  and  Female  Gametes. — The  wide  difference  in 
structure  and  function  between  the  male  and  female 
gametes  of  Volvox  globator  is  repeated  in  the  sexual 
differentiation  of  gametes  in  the  vast  majority  of 
organisms,  including  all  the  higher  forms  of  life,  both 


CHARACTERS   OF   MALE   AND   FEMALE   GAMETES     201 

animals  and  plants,  and  in  nearly  all  the  higher  organisms 
— Seed  Plants  and  a  few  other  are  exceptions — the 
male  gamete  or  sperm  is  a  small  motile  free  swim- 
ming cell,  while  the  female  gamete  or  egg  is  a  large 
passive  spherical  cell.  The  sperm  is  exceedingly  sensi- 
tive, at  least  in  a  large  number  of  cases,  to  chemical 
substances  diffusing  out  from  the  female  cell.  Sperms 
are  produced  in  immense  numbers  and  only  a  minute 
proportion  succeed  in  conjugating  with  eggs.  The  body 
of  the  sperm  cell  is  reduced  to  the  smallest  size  compatible 
with  its  function,  which  is  to  carry  the  paternal  nucleus 
to  the  egg.  Its  chromatin  contribution  to  the  zygote 
nucleus  is  exactly  equal  to  that  of  the  egg  nucleus 
(see  Fig.  35,  F,  G).  The  hereditary  characters  are 
carried  by  the  conjugating  nuclei,  and  the  paternal 
and  maternal  chromatin  are  equal  in  bulk  and  equiva- 
lent in  effect  on  the  characters  of  the  offspring. 

The  egg,  on  the  other  hand,  provides  in  the  first 
instance  the  store  of  organic  food  substance  with 
which  the  new  individual  produced  from  the  zygote 
starts  its  life,  and  the  size  and  passivity  of  the  female 
gamete  are  correlated  with  this  function.  The  fact 
that  far  fewer  eggs  than  sperms  are  produced  is  also 
a  result  of  this  difference  in  size.  The  first  beginnings 
of  the  differentiation  we  saw  in  Chlamydomonas  and 
in  Pandorina.  The  comparatively  slight  difference 
between  the  gametes  in  these  forms  appears  as  the 
result  of  what  may  be  regarded  as  an  accidental  differ- 
ence in  the  rapidity  and  duration  of  the  process  of 
division  of  the  mother  cell.  The  divisions  of  the 
mother  cell  of  the  smaller  (male)  gametes  are  more 
rapid  and  continue  longer,  so  that  smaller  cells  are 
produced,  while  the  divisions  of  the  mother  cell  of 
the  female  gametes  is  slower,  so  that  larger  cells  are 


202  ORIGIN   OF   SEX   AND   OF   THE   SOMA 

produced,  the  structure  of  the  two  being  identical. 
The  individual  sluggishness  of  the  larger  gametes  in 
Pandorina  is  probably  merely  a  result  of  their  larger 
size,  but  since  they  tend  to  remain  passive  and  also 
contribute  more  cytoplasm  to  the  zygote,  we  see  here 
the  foundations  laid  of  the  specific  characters — the 
femaleness — of  the  egg  in  the  higher  forms.  In  these 
the  differentiation  is  carried  further  and  has  become 
fixed — the  smaller  (male)  gametes  are  no  longer  capable 
of  conjugating  with  one  another.  They  are  short- 
lived cells,  incapable  of  nourishing  themselves,  but 
extremely  active  and  sensitive,  their  sole  function 
being  to  get  to  the  egg  as  quickly  as  possible  and 
contribute  their  quota  of  nuclear  material  to  the 
zygote.  The  female  gamete  has  taken  over  entirely 
the  function  of  feeding  the  new  individual,  and  the 
amount  of  food  that  can  be  stored  in  it  is  not  limited 
by  the  need  of  motility.  This  is  a  much  more  efficient 
arrangement  for  giving  the  new  individual  a  good 
start  in  life  than  the  conjugation  of  isogametes,  both 
of  which  are  motile  and  neither  of  which  can  con- 
tribute much  food  to  the  zygote. 

The  origin  of  sex  is  an  excellent  example  of  the 
origin  of  a  differentiation  which  is  at  first,  as  it  were, 
accidental,  i.e.  appearing  without  reference  to  its  ulti- 
mate use,  but  is  later  fixed  and  further  developed  into 
an  extremely  efficient  working  mechanism.  The  more 
we  learn  of  the  evolution  of  structure  and  function  in 
organisms  the  more  we  find  that  something  like  this 
is  the  history  of  the  evolution  of  new  characters. 

In  the  higher  forms  the^egg  is  often,  as  we  shall  see 
in  the  sequel,  much  reduced  in  size  because  parts  of 
the  parent  organism  take  over  from  the  egg  the 
function  of  providing  food  for  the  new  individual 
produced  from  the  zygote. 


FILAMENTOUS   GREEN   ALG2E  20$ 

Besides  the  motile  green  algae,  some  of  which  have 
been  described  in  the  preceding  pages,  there  are  many 
immotile  unicellular  forms,  such  for  instance  as  Proto- 
coccus  (Chapter  IV).  Some  of  these  live  on  damp 
earth,  tree-trunks,  or  in  similar  damp  situations, 
while  many  float  in  water.  Some  are  ccenobiate,  like 
the  Volvocines,  and  others  form  irregular  loose  colonies 
with  an  indefinite  number  of  cells. 

But  there  are  also  a  large  number  of  filamentous 
(thread-like)  forms,  the  body  commonly  consisting  of 
simple  (Fig.  30,  a)  or  branched  (g)  threads  composed  of 
cylindrical  cells  placed  end  to  end  and  containing  one 
or  more  chloroplasts  of  various  shapes.  Many  of  the 
filamentous  forms  which  live  in  water  are  reproduced 
by  the  division  of  the  contents  of  their  cells  into 
motile  flagellate  cells  called  zoospores  (Fig.  30,  6),  of  the 
same  type  of  structure  as  the  Chlamydomonas  cell, 
but  smaller  and  without  a  cell  wall — cells  in  fact 
closely  similar  to  Chlamydomonas  gametes.  These 
zoospores  escape  from  the  mother  cell  in  which  they 
were  formed,  swim  about  for  a  tmie  (c),  and  then  settle 
down  on  some  solid  object  and  germinate,  secreting  a 
cell  wall  (d),  growing  in  length  (e)  and  dividing  to 
form  a  thread  of  cells  of  the  type  characteristic  of 
the  species  (Fig.  30,  /).  We  may  consider  that  in  this 
form  of  reproduction  the  plant  reverts  to  the  condi- 
tion of  a  Chlamydomonas-]ike  ancestor  for  the  purpose 
of  reproduction  :  or  to  put  the  matter  another  way  a 
Chlamydomonas-like  ancestor  settled  down  and  divided 
without  separation  of  the  daughter  cells  from  the 
cell  wall,  and  then  by  growth  and  further  division  a 
thread  of  cells  was  produced,  any  of  which  at  a  later 
stage  can  produce  a  brood  of  Chlamydomonas-like 
cells  which  escape,  and  each  of  which  reproduces  the 
filament. 


204 


ORIGIN   OF   SEX   AND   OF  THE   SOMA 


The  zoospores  of  such  a  filamentous  alga  may  in 
certain  cases  conjugate  instead  of  germinating  at  once, 
thus  acting  as  isogametes.  Such  zoospores  are  called 
facultative  gametes.  But  in  most  cases  the  gametes  are 


FIG.  30. — Filamentous  green  algae  :  a,  vegetative  thread  of  Ulothrix 
with  band-shaped  chloroplasts  (chl.)  ;  b,  thread  whose  cells 
are  forming  zoospores  (like  the  individuals  of  Chlamydomonas 
but  without  walls)  ;  c,  Free-swimming  zoospore  ;  d,  germination 
of  zoospore,  which  has  lost  its  flagella,  is  attached  by  the  front 
end  to  a  solid  object  and  has  secreted  a  wall  ;  e,  the  cell  has 
grown  in  length  ;  /,  young  filament  derived  from  e  by  further 
growth  in  length  and  transverse  cell  division  (this  grows  into 
a  filament  like  a)  ;  g,  branched  filamentous  alga  (Stigeoclonium), 
some  of  whose  cells  are  vegetative,  others  have  formed  zoospores  ; 
h,  free  zoospores. 

smaller  than  the  zoospores,  though  quite  similar  in 
structure,  i.e.  more  of  the  gametes,  often  double  as 
many,  are  produced  from  the  mother  cell.  Some 


EVOLUTION   OF   THE   SOMA  2O5 

filamentous  forms  have  sexually  differentiated  gametes, 
and  these  in  most  cases  show  a  wide  differentiation 
between  sperms  and  eggs,  comparable  with  that  seen 
in  Volvox. 

Sexual  differentiation  of  gametes  is  found  in  various 
different  groups  among  the  algae,  evidently  inde- 
pendently evolved  along  separate  lines  of  descent. 
One  may  say  that  there  is  an  inevitable  tendency 
towards  sexual  differentiation,  and  towards  the  fixa- 
tion of  the  extreme  form  in  which  the  sperms  are 
most  widely  different  from  the  eggs.  This  high 
specialisation  of  the  gametes,  as  already  shown,  is 
certainly  the  most  efficient  mechanism  for  the  pro- 
duction of  vigorous  new  individuals  through  conjugation. 

The  "  Soma  "  in  Plants  and  Animals. — The  evolu- 
tion of  a  soma  or  mortal  body  is  beautifully  illustrated 
in  the  Pandorina-Eudorina-Pleodorina-Volvox  series. 
This  evolution  consists  essentially  in  the  separation  of 
the  vegetative  and  reproductive  functions.  In  the 
lower  forms  of  the  series  all  the  cells  of  the  body  dis- 
charge both  functions,  in  the  higher  some  cells  dis- 
charge the  vegetative,  others  the  reproductive  func- 
tion. Directly  we  have  any  cells  limited  to  the  vege- 
tative functions,  we  have  a  soma  or  mortal  "  body  " 
by  the  very  fact  that  these  cells  can  no  longer  reproduce 
the  species. 

It  must  be  noted,  however,  that  this  particular  series 
of  organisms  is  not  in  the  direct  line  of  evolution  of 
any  of  the  higher  organisms.  Volvox  is  the  culmi- 
nation of  its  own  line  of  descent.  It  is  probable  that 
no  further  increase  in  size  or  complication  is  possible 
to  the  motile  ccenobiate  form  of  organism.  The  soma 
has  been  evolved  on  many  other  lines  of  descent  from 
unicellular  organisms.  This  particular  line  is  chosen 


206  ORIGIN   OF  SEX   AND   OF  THE   SOMA 

for  illustration  because  of  the  existence  of  several  forms 
which  make  up  a  closely  connected  series  ;  and  mid- 
way in  this  series  Pleodorina  shows  the  actual  first 
appearance  of  the  soma,  P.  illinoiensis  being  a  very 
slight  modification  of  the  Eudorina  type. 

The  vast  majority  of  multicellular  animals  have  a 
well-marked  soma,  i.e.  a  body  consisting  of  tissues  whose 
cells  are  not  germ  cells  and  do  not  reproduce  the 
species  by  spore  or  gamete  formation  ;  but  in  many 
of  the  lower  invertebrates  new  individuals  are  produced 
by  budding  of  these  somatic  tissues  which  are  not  too 
highly  specialised  for  particular  vegetative  functions. 
In  the  higher  animals  this  process  falls  into  abeyance, 
and  the  life  of  the  somatic  tissues  is  strictly  limited  to 
the  service  of  the  individual  and  comes  to  an  end 
with  the  life  of  the  individual. 

In  plants,  however,  the  power  of  reproducing  the 
species  is  much  more  often  retained  by  the  cells  of 
the  vegetative  body.  In  the  filamentous  green  algae, 
for  instance,  referred  to  in  the  preceding  section,  all, 
or  nearly  all,  the  cells  of  the  multicellular  thread  of 
which  the  body  is  composed  retain  the  power  of  form- 
ing zoospores  and  gametes,  and  thus  may  become 
"  germ  cells."  In  the  bulky  algae  (seaweeds)  with 
massive  tissues,  as  well  as  in  all  the  higher  plants,  this 
power  is  lost  by  the  general  body  cells,  but  the  plant 
may  be  reproduced  "  vegetatively  "  by  budding,  as  in 
many  of  the  lower  invertebrate  animals,  new  plants 
being  thus  formed  apart  from  the  germ  cells  proper. 
This  power  of  "  vegetative  reproduction  "  is  retained 
by  some  at  least  of  the  body  cells  of  practically  all 
plants,  even  the  highest  and  most  complicated  forms, 
as  we  shall  see  in  later  chapters. 

Taken  together  these  facts  show  us  that  the  dis- 


SPIROGYRA  2O7 

tinction  between  somatic  and  germ  cells  is  not  an 
absolute  one.  Though  the  germ  cells  (spores  and 
gametes)  are  the  specialised  reproductive  cells,  whose 
sole  function  is  to  reproduce  the  species,  this  power 
may  be  retained  by  the  body  cells  to  a  varying  extent 
in  different  organisms.  The  body  cells  of  plants  retain 
it  far  more  generally  than  those  of  animals,  and  this 
is  undoubtedly  connected  with  the  fact  that  plant 
cells  are,  in  general,  far  less  highly  modified  for  the 
performance  of  special  functions  than  is  the  case  with 
the  cells  of  the  higher  animals. 

Spirogyra. — This  is  a  filamentous  green  alga  belong- 
ing to  a  group  which  is  distinguished  by  producing 
gametes  that  are  not  flagellated  cells,  conjugation 
taking  place  entirely  within  the  mother  cell  walls.  The 
body  consists  of  an  unbranched  thread  composed  of 
cylindrical  cells  placed  end  to  end.  Each  cell  has  a 
large  central  vacuole  and  a  thin  layer  of  cytoplasm 
lining  the  wall :  in  this  is  embedded  a  single  chloro- 
plast  in  the  form  of  a  green  band  which  winds 
spirally  round  the  cell  from  end  to  end  (Fig.  31,  A). 
In  the  chloroplast  is  a  row  of  pyrenoids.  The  nucleus 
is  suspended  in  the  centre  of  the  cell.  In  other  species 
several  chloroplasts  are  present,  running  parallel  with 
one  another.  The  crossing  lattice  structure  seen  when 
the  cell  is  looked  through  from  the  side  is  due 
to  the  fact  that  the  parts  of  the  chloroplasts  which 
run  round  the  further  side  of  the  cell  are  seen  at  the 
same  time  as  the  parts  on  the  side  towards  the  observer, 
so  that  the  former  appear  to  cross  the  latter.  In  a 
wide  cell  containing  several  chloroplasts,  when  the 
near  side  of  the  cell  can  be  focussed  alone,  the  far 
side  being  out*of  view,  it  is  clear  that  there  is  no  actual 
crossing  (Fig.  31,  B).  In  such  a  large  cell  the  nucleus 


208  ORIGIN   OF   SEX   AND   OF  THE   SOMA 


FIG.  31. — Spirogyra.  A,  one  complete  cell  with  parts  of  two  adjacent 
cells  of  the  filament  of  a  species  with  one  chloroplast  in  each  cell ; 
cy.,  cytoplasm  lining  the  wall ;  v,  vacuole ;  n,  nucleus  suspended 
by  a  group  of  bridles;  cA/.,  spiral  chloroplast;  p,  pyfenoid.  B, 
surface  view  of  cell  of  large  species  with  several  chloroplasts, 
showing  nucleus  (n)  lying  below,  connected  with  surface  by 
bridles  (br.)  which  run  to  pyrenoids  (p).  X  about  550.  C,  zygote 
formed  by  conjugation  of  gamete  formed  in  neighbouring  cell 
(marked  <£  =  male)  of  the  same  filament  with  gamete  formed 
in  cell  marked  $  =  female.  D,  five  filaments  which  have 
taken  part  in  conjugation.  On  the  extreme  left  filament  which 
has  acted  as  female,  and  all  of  whose  cells  contain  zygotes.  Next 
is  a  filament  all  of  whose  cells  have  acted  as  male,  alternate  cells, 
conjugating  with  those  of  the  filament  on  the  left  and  on  the 
right.  On  the  extreme  right  are  two  filaments  in  process  of  con- 
jugation— successive  stages  from  above  downwards  (left,  male  ; 
right,  female).  At  the  top  of  the  right-hand  filament  a  cell  (v.c.) 
which  has  remained  vegetative.  (Modified  from  G.  S.  West. 


SPIROGYRA  209 

is  suspended  by  long  thin  bridles  (br.),  each  of  which 
runs  into  a  chloroplast  opposite  a  pyrenoid  (p).  This 
is  probably  connected  with  the  function  of  the  nucleus 
in  controlling  nutrition  (see  p.  67),  since  it  is  round  the 
pyrenoids  that  starch  is  laid  down. 

The  cells  of  Spirogyra  divide,  after  karyokinetic 
division  of  the  nucleus,  exclusively  in  the  plane  per- 
pendicular to  the  long  axis  and  halfway  between  the 
end  walls,  the  new  cell  wall,  secreted  and  constantly 
covered  by  cytoplasm,  growing  out  in  the  form  of  a 
ring  from  the  cylindrical  wall,  the  effect  being  like 
the  gradual  closing  of  an  iris  diaphragm,  till  the  two 
daughter  cell  cavities  are  completely  separated.  The 
cells  then  grow  in  length  till  they  reach  the  length  of 
the  standard  cell  of  the  species.  Under  certain  con- 
ditions the  thread  breaks  up  into  lengths,  or  even  into 
single  cells,  by  the  splitting  apart  of  adjacent  cells  ; 
and  in  this  way  the  number  of  individual  threads 
increases.  Spirogyra  is  sometimes  said  to  be  "  physio- 
logically unicellular,"  because  each  cell  functions  as 
a  self-contained  unit — it  is  immaterial  to  its  life 
whether  it  is  isolated  or  whether  it  forms  part  of  a 
thread. 

Gamete  Formation  and  Conjugation. — Under  certain 
conditions  two  threads  lying  side  by  side  form  gametes, 
one  from  each  cell.  The  cell  walls  of  one  of  the  threads 
on  the  side  towards  the  other  thread  are  thrust  out 
in  blunt  projections,  one  from  each  cell,  and  the  cell 
body  of  each  cell  begins  to  leave  the  wall.  Almost 
immediately  similar  projections  are  thrust  out  from 
the  cells  of  the  other  thread  opposite  those  of  the 
first  set,  the  pairs  of  projections  meet  between  the 
threads,  the  parts  of  the  cell  walls  in  contact  are 
absorbed,  and  an  open  conjugation  canal  is  thus  formed 
14 


2IO  ORIGIN   OF   SEX   AND   OF   THE   SOMA 

between  the  two  cell  cavities.  The  cell  bodies  of  the 
thread  which  began  the  process  (male  gametes)  now  slip 
through  the  canals  and  fuse  with  the  bodies  of  the 
opposite  cells  of  the  other  thread  (female  gametes), 
which  have  meanwhile  contracted  away  from  the  cell 
wall,  and  a  zygote  is  thus  formed  in  the  cavity  of 
each  female  cell  (Fig.  31,  D).  The  zygote  becomes 
covered  by  a  thick  wall  and  ultimately  germinates  to 
form  a  new  Spirogyra  thread  by  splitting  of  the  thick 
outer  wall  and  protrusion  of  the  inner  thin  wall 
(Fig.  32,  C).  Cf.  the  germination  of  the  zygote  of 
Mucor  (p.  163,  and  Fig  16,  i).  In  some  species,  however, 
adjacent  cells  of  the  same  thread  may  conjugate 
(Fig.  31,  C),  one  acting  as  the  male  the  other  as  the 
female  cell,  and  in  other  cases,  again,  parthenospores  may 
be  formed,  quite  similar  to  zygotes  in  appearance  and 
germination,  but  each  produced  from  the  contents  of  a 
single  cell  without  conjugation. 

The  sexual  differentiation  of  the  gametes  of 
Spirogyra  is  mainly  seen  in  the  active  and  passive 
roles  of  the  two  conjugating  cells  :  it  does  not  involve 
a  difference  of  structure  or  even  of  size.  But  in  one 
species  at  least  there  is  an  interesting  change  in  the 
structure  of  the  male  gamete  after  it  becomes  part  of 
the  zygote,  its  chloroplast  degenerating  and  disin- 
tegrating in  the  zygote  (Fig.  32,  A,  B),  so  that  only  the 
chloroplast  derived  from  the  female  gamete  remains 
and  gives  rise  to  the  chloroplasts  of  the  new  thread 
formed  on  germination.  This  is  a  late  occurring 
degeneration  of  the  nutritive  equipment  of  the  male 
gamete,  which  may  be  compared  with  the  reduction 
or  disappearance  of  the  chloroplast  in  the  ordinary 
sperm  before  the  formation  of  the  gamete  (cf.  p.  200). 
Other  slight  indications  of  "  maleness  "  may  be  found 


DIFFERENTIATION   OF   GAMETES 


211 


in  some  species,  for  instance  the  cells  forming  male 
gametes  are  sometimes  shorter  than  those  forming  the 


FIG.  32. — A  and  B,  two  stages  of  development  of  the  zygote  of  a 
species  of  Spirogyra  in  which  the  wall  is  transparent  and  the 
chloroplast  of  the  male  gamete  is  degenerating  and  breaking 
up.  The  chloroplast  of  the  female  gamete  alone  produces  the 
chloroplasts  of  the  cells  of  the  new  individual,  x  800  (after 
Chmielevsky) .  C,  germination  of  zygote  :  two  cells  of  the  new 
individual  are  produced.  X  395. 


females,  and  in  other  cases  the  conjugation  canal  is 
mainly  or  even  wholly  formed  by  the  male  conjugating 
cell,  the  female  cell  merely  swelling  up  to  meet  it. 


212  ORIGIN    OF    SEX    AND    OF    THE    SOMA 


PRACTICAL  WORK.' 

(1)  In  a  sample  of  Chlamydomonas  note,  under  the  low  power, 
the  moving  green  dots.     Under  the  high  power  note  in  a  specimen 
at  rest   (the  flagella  often  get  stuck  to  the  slide  or  coverslip, 
and  the  cell  thus  anchored  oscillates  to  and  fro)  the  cell  wall, 
the  basin  shaped  chloroplast  with  pyrenoid,  the  clear  front  end 
of  the  body  with  the   base  of  the  flagella,  and  the  red  eyespot. 
Look  out  for  stages  of  division.     Run  a  drop  of  dilute  iodine 
solution  under  the  coverslip  and  observe  again. 

(2)  In   a  living  demonstration  specimen   under  an   apochro- 
matic  immersion  lens  observe  the  finer  details  of  cell  structure, 
including  the  nucleus  situated  in  the  colourless  central  protoplasm 
in  the  hollow  of  the  chloroplast. 

(3)  Examine  if  possible  Pandorina  and  Eudorina  (preserved 
material  if  fresh  cannot  be  obtained),  noting  the  construction  of 
the  coenobium  and  the  fact  that  the   structure  of  each  cell  is 
of  the   Chlamydomonas   type.     Examine  also  stages  of  division 
to  form  daughter  coenobia  if  these  are  available. 

(4)  Volvox.     In    V.    aureus    (on    the    whole    the    commonest 
species)   note  the  large  spherical  ccenobia,   each  consisting  of 
several   hundred   spherical    cells.     Compare   several   specimens, 
and  trace  the  development  of  the  daughter  ccenobia  from  the 
large  cells  (parthenogonidia)  of  the  mother.     If  sexual  colonies 
are  available  note  the  development  of  the  sperms  by  repeated 
division  of  special  cells  (androgonidia)  and  the  large  gynogonidia, 
each  of  which  becomes  an  egg. 

(5)  Examine  a  single  vegetative  cell  under  the  high  power 
and  note  that  it  is  of   the  Chlamydomonas  type.     Look  for  the 
flagella.     Run  in  a  drop  of  iodine  and  look  for  the  threads  of 
cytoplasm  connecting  the  cells. 

(6)  Compare  the  structure  of   V.  globator,    and   examine  any 
demonstration  slides  of  stages  of  development  and  details  of 
cell  structure  that  may  be  available. 

1  The  material  for  this  practical  work  may  be  varied  according 
to  what  can  be  obtained.  Chlamydomonas  is  generally  available 
during  the  warmer  months,  and  can  in  any  case  be  kept  in  the  labora- 
tory without  much  difficulty.  The  ccenobiate  forms  are  not  so  easy 
to  keep  in  cultivation,  and  fresh  material  is  by  no  means  always 
available.  For  this  reason  it  is  advisable  to  keep  a  stock  of  material 
preserved  in  formalin,  illustrating  at  least  the  vegetative  structure 
of  Pandorina,  Eudorina,  Pleodorina,  and  Volvox,  and  the  formation 
of  daughter  coenobia  in  these  forms.  Pleodorina  can  often  be  found 
by  careful  searching  through  samples  of  Eudorina.  It  is  desirable 
also  to  prepare  slides  showing  the  formation  of  gametes,  etc. 


PRACTICAL   WORK  213 

SPIROGYRA. 

(7)  Draw  a  single  cell  '  on  a  large  scale  under  the  high  power, 
showing  cell  wall,  cytoplasm,  chloroplast  with  pyrenoids,  vacuole, 
nucleus  (with  nucleolus)  and  bridles  (if  present). 

(8)  Compare  the  pyrenoids  of   two  preserved  and  decolorised 
samples,   one    of    which,    previous    to    killing    has    been    well 
illuminated,  the  other  kept  for  a  day  or  two  in  the  dark.     Now 
stain  each  with  iodine  and  observe  again.     Note  that  the  pyrenoid 
itself  stains  brown  with  iodine   (it  is  a  protein  crystalloid)  as 
contrasted  with  the  starch  formed  round  it  which  stains  dark 
blue  (almost  black). 

(9)  Examine  the  conjugation  of  Spirogyra,  fresh  if  possible,  if 
not  in  preserved  material,  and  draw  as  many  stages  in  the  process 
of  conjugation  as  you  can  find. 

(10)  Plasmolyse  vegetative  Spirogyra  *  with  5  per  cent,  calcium 
chloride  solution  (ordinary  salt  solution  injures  the  cells)  and  draw 
stages  in  plasmolysis.     Recover  by  placing  in  water. 

1  A  species  with  a  single  chloroplast  or  two  chloroplasts  in  each 
cell  is  the  easiest  to  draw  in  the  first  instance.  This  should  be  com- 
pared with  a  large  species  such  as  S.  crassa  containing  seven  or  eight 
chloroplasts,  and  showing  bridles  running  into  the  pyrenoids. 

J  If  time  permits. 


CHAPTER    XIII 

DIFFERENTIATION  OF  TISSUES.     FUCUS  :  THE 
SEA-WRACK 

BESIDES  the  Green  Algae,  which  are  mostly  unicellular  or 
filamentous  fresh- water  forms  (the  "  sea  lettuce  "  is  an 
example  of  a  green  alga  which  is  marine  and  has  a  large 
thin  body  or  thallus,  which  is  soft  and  membranous, 
and  composed  of  two  layers  of  green  cells),  there  are 
two  other  large  groups  of  algae,  commonly  called  the 
Red  and  the  Brown  Seaweeds,  because  they  are 
respectively  red  and  brown  in  colour  and  they  live  in 
the  sea,  mostly  in  the  intertidal  zone  or  not  very  far 
below  low-water  mark.  The  plastids  of  the  cells  of  these 
seaweeds  contain  the  chlorophyll  pigments,  but  in  a 
different  proportion  to  that  in  which  they  exist  in 
green  chlorophyll,  and  also  other  pigments  in  addi- 
tion. For  instance  in  the  Brown  Seaweeds  there  is  a 
smaller  proportion  of  the  pure  green  pigments  (chloro- 
phyll a  and  /?)  and  a  larger  proportion  of  the  yellow 
and  orange  constituents  (xanthophyll  and  carotin), 
and,  in  addition,  a  special  orange  pigment,  fucoxanthin 
(C40H54O6),  peculiar  to  the  Brown  Seaweeds.  This 
combination  gives  the  phceoplasts ,r  as  they  are  called, 
a  brown  or  olive  green  colour. 

The  Brown  Algae  vary  from  unicellular  forms,  through 
a  considerable  series  of  filamentous  types,  either  con- 

1  Greek  <f>aio<;,  greyish. 
314 


EXTERNAL   STRUCTURE   OF   FUCUS  215 

sisting  of  single  rows  of  cells  or  of  stouter  threads 
several  cells  thick,  to  large  forms  :  with  quite  bulky 
bodies  consisting  of  more  or  less  differentiated  tissues. 
One  of  these  latter  forms  we  shall  now  study  in  some 
detail,  because  it  illustrates  very  well  the  fundamental 
principle  of  differentiation  of  function  and  corresponding 
differentiation  of  structure  leading  to  the  origin  of 
distinct  tissues. 

Several  different  species  of  the  genus  Fucus  live 
attached  to  rocks,  mainly  between  tide  marks,  on 
the  coasts  of  the  cooler  countries  of  the  northern 
hemisphere.  They  are  the  commonest  seaweeds  on  the 
rocky  British  coasts,  and  frequently  cover  the  rocks  so 
thickly  as  to  make  them  very  slippery  to  walk  upon 
just  after  the  tide  has  ebbed,  leaving  the  mucilaginous 
surface  of  the  seaweed  wet  and  slimy. 

The  individual  plants  of  Fucus  vary  from  a  few 
inches  to  several  feet  in  length.  The  body  or  thallus 
consists  of  a  cylindrical  stalk  or  stipe  attached  to  the 
rock  substratum  by  a  more  or  less  branched  spread- 
ing holdfast  which  cements  itself  firmly  by  means  of 
the  gluelike  mucilaginous  walls  of  the  surface  cells 
to  the  rock  or  stone  on  which  the  plant  grows.  Above, 
the  stipe  passes  into  the  flat-forked  frond  by  the  gradual 
appearance  of  thin  wings  distinct  from  the  thickened 
midrib,  which  is  a  direct  continuation  of  the  stipe.  At 
the  end  of  each  branch  of  the  frond  is  a  groove  or 
depression,  often  containing  hairs  consisting  of  single 
chains  of  cells,  and  it  is  by  division  of  the  cells  at  the 
base  of  the  groove  (Fig.  33,  A)  that  the  branches  grow. 

The  end  portions  of  some  of  the  branches  are 
swollen,  and  thickly  set  in  the  swollen  frond  are  slightly 

1  Some  of  the  Brown  Seaweeds,  for  instance  Macrocystis  and 
Nereocystis,  living  in  the  Antarctic  and  Pacific  Oceans,  are  immense 
plants,  hundreds  of  feet  long. 


2l6  DIFFERENTIATION   OF  TISSUES.      FUCUS 

raised  papillae,  each  of  which  is  the  projecting  top  of 
a  spherical  structure,  hollow  within,  called  a  con- 
ceptacle.  A  minute  hole  leads  through  the  tip  of  the 
papilla  into  the  hollow  of  the  conceptacle,  which  con- 
tains the  sexual  organs  bearing  the  highly  differentiated 
gametes.  There  are  also  (often)  smaller,  sterile  con- 
ceptacles  from  which  hairs  protrude  through  the 
opening  to  the  outside. 

Thus  in  Fucus  we  have  an  external  differentiation 
of  parts  or  organs,  though  a  very  simple  one  :  the 
holdfast  which  fixes  the  plant  below,  the  stipe  which 
is  specially  tough,  and  the  frond  which  plays  the  chief 
part  in  photosynthesis  and  in  growth ;  while  the 
sexual  reproductive  organs  are  represented  by  groups  of 
cells  arising  on  what  is  really  the  surface  of  the  frond, 
though  the  surface  locally  dips  down,  so  to  speak,  to 
form  the  internal  surface  of  the  hollow  conceptacle. 

Microscopic  Structure  of  the  Thallus. — The  minute 
structure  of  the  different  organs  of  bulky  plants  is 
best  studied  by  examining  under  the  microscope 
sections  of  the  organ  thin  enough  to  be  translucent 
when  mounted  in  a  liquid  medium.  To  obtain  com- 
plete information  as  to  the  structure  of  such  an  organ 
these  sections  have  to  be  cut  in  different  directions.  The 
most  instructive  section  is  that  taken  at  right  angles 
to  the  axis  of  symmetry  of  the  organ  (transverse  or 
cross  section),  for  this  displays  the  distribution  of 
tissuer  about  that  axis.  But  the  knowledge  we  gain 
from  a  transverse  section  must  be  supplemented  by 
the  examination  of  sections  taken  through  and  parallel 
with  the  axis  (longitudinal  sections),  in  order  to  study 
the  structure  of  the  tissues  in  longitudinal  extension. 

(i)  Frond. — A  cross-section  of  the  middle  of  the 
frond  shows  three  clearly  marked  regions :  (a)  the 


INTERNAL   STRUCTURE  217 

surface  layer  of  cells  (palisade  or  photo  synthetic  layer), 

(b)  the  larger  isodiametric  cells  lying  below  (cortex),  and 

(c)  the  central  region  of  cells    (medulla)   the  bodies  of 
which  are  separated  from  one  another  by  more  than 
the  thickness  of  an  ordinary  cell  wall. 

(a)  Palisade  Layer. — This  consists  of  a  single  layer  of 
cells  (Fig.  33,  A,  p)  whose  long  axes  are  perpendicular  to 
the    surface    of    the    frond.     Each    cell    of    this    layer 
essentially    resembles    a    mesophyll    cell    (especially    a 
palisade  cell)  of  the  leaf  of  a  higher  plant  (Figs.  10,  n). 
It  contains  a  central   vacuole   and   a  peripheral  layer 
of  cytoplasm  containing  the  nucleus  and  packed  with 
phaeoplasts.     In  this  layer  (as  in  the  palisade  layer  of 
the  mesophyll  of   a  typical  leaf)  the  greater   part  of 
the  work  of  photosynthesis  is  carried  on.     These  cells 
contain  the  greatest  mass  of  phaeoplasts,  and  the  raw 
materials    of    the    process    (water,    dissolved    carbon 
dioxide  and  mineral  salts)  have  direct  access  to  them 
when  the  plant  is  covered  by  the  sea  at  high  tide. 

(b)  Cortex. — These  cells  (Fig.  33,  A,  c)  are  larger  than 
those  of  the  palisade  layer,  having  larger  vacuoles  and 
fewer  phaaoplasts  per  unit  bulk.     The  nucleus  is  often 
suspended  in  the  vacuole  by  cytoplasmic  bridles. 

(c)  Medulla. — The    central    region    of    the    frond    is 
occupied  by  cells  (Fig.  33,  A,  me.)  most  of  which  are 
apparently  isolated  from  one  another.     They  are  not, 
however,  separated   by   air   spaces   like   many  of   the 
cells  of  the  tissues  of  a  higher  plant,  but  by  a  mucila- 
ginous substance  (mu.),  which  is  really  formed  by  the 
swelling  of  the  middle  layer  of  the  joint  wall  between 
two  adjacent  cells.     When  these  cells  are  first  formed 
in  development,  the  cell  bodies  are  separated  by  thin 
walls,   but   the   walls  gradually  increase  in   thickness 
and  the  middle  layer  becomes  mucilaginous,  takes  up 


2l8  DIFFERENTIATION   OF  TISSUES.      FUCUS 

water  and  swells,  forcing  the  cells  apart  and  increasing 
the  thickness  of  the  thallus.  The  layers  of  wall  on  each 
side  of  this  swollen  middle  layer,  i.e.  in  direct  contact 
with  the  cell  bodies,  also  increase  in  thickness,  more 
or  less,  but  remain  of  firmer  consistency,  so  that  in 
the  adult  condition  the  cells  appear  isolated,  each 
covered  by  a  wall  of  its  own  (which  may  be  thin  or 
thick)  and  separated  from  its  neighbours  by  a  mucila- 
ginous matrix.  The  medullary  cells  form  chains  or 
strands  (like  the  threads  of  a  filamentous  alga)  of 
cylindrical  cells  placed  end  to  end.  It  is  these  chains 
which  are  separated  from  one  another  by  the  mucila- 
ginous matrix  derived  from  the  middle  layer  of  the 
original  cell  wall  (Fig.  33,  A).  The  cross  walls  separating 
the  successive  cells  of  a  medullary  strand  remain  thin. 
The  structure  of  the  body  of  a  medullary  cell  is  not 
different  in  essentials  from  that  of  a  cortical  cell,  that 
is  to  say  there  is  a  central  vacuole  and  peripheral 
cytoplasm  with  nucleus  and  phaeoplasts,  but  these 
last  are  often  very  sparsely  scattered. 

The  direction  of  the  medullary  strands  differs  as 
between  the  wings  and  the  midrib  :  in  the  former  they 
run  horizontally  or  obliquely,  in  the  latter  longitudin- 
ally, so  that  they  appear  in  cross-section  as  circles. 
Groups  of  cells  at  the  outer  edge  of  the  medulla  of 
the  midrib,  just  below  the  cortex,  have  especially 
thick  tough  walls,  and  these  may  be  called  fibres,  by 
analogy  with  the  somewhat  similar  longitudinally 
running  thick  walled  cells  of  the  higher  plants.  Like 
them  they  increase  the  toughness  of  the  thallus.  The 
main  function  of  the  medullary  cells  is  probably  that 
of  conduction  of  organic  food  substances  lormed  by 
the  photosynthetic  layer  to  the  regions  of  growth  at 
the  apex  of  the  frond. 


STRUCTURE    OF   STIPE    AND   APEX  2IQ 

In  longitudinal  sections  of  the  frond  the  palisade 
and  cortical  cells  appear  very  much  the  same  as  they 
do  in  transverse  sections,  but  the  medullary  cells  of 
course  look  different,  those  of  the  midrib  being  now 
cut  in  longitudinal  section,  i.e.  showing  the  length  of 
the  chains,  instead  of  as  single  cells  cut  transversely. 
(2)  Stipe. — The  stipe  consists  of  cells  similar  to  those 
of  the  frond,  but  with  some  notable  differences.  There 
is  no  single  layer  of  surface  cells  overlying  a  distinct 
cortex,  but  several  layers  of  cells  in  radial  rows,  the 
cells  of  each  row  separated  by  thin  walls,  indicating 
division  parallel  to  the  surface  of  the  frond  (tangential 
division).  The  surface  itself  is  rough  owing  to  the 
knocking  about  the  stipe  suffers  from  the  waves 
against  the  rocks,  and  often  shows  radial  splits.  The 
medulla,  which  forms  much  the  greatest  bulk  of  the 
stipe,  consists  largely  of  fibres,  with  a  few  wide  cells 
having  dense  vacuolated  contents,  including  a  number 
of  phaeoplasts  and  fairly  thick  walls.  These  may 
perhaps  serve  to  conduct  organic  food  from  the  frond 
down  to  the  holdfast,  which  increases  in  size,  sending 
out  fresh  short  branches  as  the  plant  grows.  As  the 
base  of  the  stipe  is  approached  the  thick-walled  fibres 
increase  and  the  holdfast  itself  consists  exclusively  of 
them. 

Apical  Growth  and  Nutrition.— The  basis  of  growth 
in  Fucus,  as  in  the  higher  plants,  is  active  cell  divi- 
sion at  the  tips  of  the  branches.  At  the  bottom  of 
the  apical  groove  of  each  branch  there  is  situated  a 
comparatively  large  six-sided,  box-like  cell  (Fig.  33,  A,  a), 
with  one  side  forming  part  of  the  surface  of  the  base 
of  the  groove.  This  apical  cell  gives  rise  by  its  divi- 
sions to  the  cells  of  all  the  tissues  of  the  branch.  The 
apical  cell  is  constantly  dividing  unequally,  cutting  off 


220  DIFFERENTIATION  OF  TISSUES.      FUCUS 

a  flattish  oblong  cell  from  each  of  the  four  sides  and 
from  the  base  (towards  the  centre  of  the  frond)  in 
turn.  After  each  division  it  grows  to  its  original 
size.  The  cells  cut  off  divide  further  and  form  an 
almost  homogeneous  small-celled  thin-walled  tissue, 
densely  filled  with  phseoplasts,  in  the  whole  region  of 
the  branch  apex.  As  one  passes  away  from  the  apex 
the  different  tissue  regions  quickly  differentiate 
(Fig-  33>  A).  The  surface  cells  do  not  change  their 
character  much  and  form  the  photosynthetic  layer. 
The  cells  lying  next  below  increase  in  size  by  swelling 
of  the  vacuole,  but  do  not  increase  their  cytoplasm. 
These  cells  become  the  cortex.  The  cells  towards  the 
centre  of  the  frond  grow  in  length  very  considerably 
and  separate  laterally  by  the  swelling  of  the  middle 
lamellae  of  their  lateral  walls,  as  already  described, 
thus  forming  the  medulla.  The  strands  of  medullary 
cells  are  also  passively  stretched  owing  to  the  elonga- 
tion of  the  frond  by  the  continued  division  of  the 
surface  cells  at  right  angles  to  the  surface  plane. 

It  is  clear  that  at  the  apices  of  the  branches  there 
is  constantly  going  on  a  great  increase  in  the  bulk  of 
protoplasm  and  of  cell  wall  substance,  and  this  requires 
a  continuous  supply  of  soluble  carbohydrates  and 
proteins  or  other  nitrogenous  organic  substances. 
Some  of  this  organic  food  will  no  doubt  be  supplied 
by  the  synthesis  of  these  substances  from  carbon 
dioxide,  water  and  salts  absorbed  by  the  surface  layers 
of  cells  close  to  the  apex,  which  contain  numerous 
phaeoplasts.  But  in  an  actively  growing  plant  the 
consumption  will  be  much  greater  than  this  supply, 
and  the  balance  required  must  come  mainly  from  the 
palisade  layer  of  the  mature  parts  of  the  frond.  The 
easiest  channels  for  this  flow  of  substances  are  neces- 


TISSUE   STRUCTURE 


221 


jrIGi  23. — A,  longitudinal  section  through  the  apex  of  frond  of  Fucus 
(the  cells  are  in  outline,  the  intercellular  mucilage  dotted);  a,  apical 
cell  surrounded  by  the  cells  produced  from  its  previous  divisions ; 
p,  photosynthetic  layer;  c,  cortex;  me.,  medullary  cells  (mu., 
mucilage  formed  from  their  walls).  Further  from  the  apex  the 
cortical  cells  increase  in  size  and  the  medullary  cells  in  length.  B, 
young  plant  of  Fucus  in  the  cylindrical  stage.  Apical  groove  just 
developing.  Medullary  cells  beginning  to  lengthen  and  to  separate ; 
rh.,  first  rhizoids  which  develop  into  the  "  holdfast."  C,  longitu- 
dinal section  of  part  of  the  bristle-like  thread  of  Stictyosiphon 
(6  cells  thick)  showing  dense  protoplasmic  contents  with  phaeoplasts 
of  surface  cells  (photosynthetic  layer),  which  are  half  the 
length  of  the  next  layer  (cortex).  These,  again,  are  half  the 
length  of  the  central  cells  (medulla),  which  are  slightly  separated 
longitudinally.  This  arrangment  is  due  to  the  more  numerous 
divisions  of  the  surface  cells.  Compare  B. 


222  DIFFERENTIATION   OF   TISSUES.      FUCUS 

sarily  the  strands  of  medullary  cells,  because  these 
form  the  shortest  paths,  and  also,  owing  to  the  length 
of  these  cells,  there  are  fewer  walls  to  pass  in  a  given 
distance.  The  medullary  cells  are  connected  at  one 
end  with  the  cortex,  and  through  it  with  the  palisade 
layer  of  the  mature  frond  ;  at  the  other  with  the  mass 
of  embryonic  cells  lying  behind  the  apical  cell. 
Hence  these  medullary  cells  may  be  called  conducting 
cells.  The  diffusion  of  substances  from  a  region  of 
higher  to  a  region  of  lower  concentration,  in  plant 
and  animal  tissues  for  instance  from  a  region  of 
constant  production  to  a  region  of  consumption  (i.e.  con- 
version into  other  substances),  is  a  necessary  conse- 
quence of  the  physical  laws  of  diffusion,  though  the 
rate  of  diffusion  and  the  actual  paths  it  takes  will 
depend  on  a  number  of  variable  factors,  such  as  the 
nature  of  the  substances  and  the  obstacles  to  diffusion. 
The  cortical  cells  will  act  to  a  certain  extent  as  a 
storage  tissue,  since  if  the  stream  of  soluble  organic 
food  from  the  photosynthetic  layer  through  the 
cortex  and  medulla  to  the  growing  points  is  checked, 
owing  to  the  supply  being  temporarily  greater  than 
the  demand,  some  of  the  surplus  food  will  be  arrested 
and  will  tend  to  accumulate  in  the  cortical  cells  as 
well  as  in  the  medullary  cells. 

Sexual  Reproduction. — Unlike  the  plants  hitherto 
considered  Fucus  reproduces  itself  exclusively  by 
means  of  gametes,  and  these  show  high  sexual  dif- 
ferentiation, comparable  with  that  of  the  gametes  of 
Volvox  (cf.  Figs.  28,  G,  and  35,  D).  The  two  kinds  of 
gametes  are  produced  in  the  cells  of  the  sexual  organs 
arising  on  the  inner  surfaces  of  the  conceptacles.  In 
some  species  the  male  and  female  organs  are  formed 
in  the  same  conceptacle  in  others  in  different  concep- 


SEXUAL   ORGANS  223 

tacles  which  may  be  produced  on  different  individual 
plants. 

The  female  organ  (oogonium)  arises  from  a  single 
cell  on  the  surface  of  the  conceptacle.  This  grows 
up  to  form  a  papilla  which  is  cut  off  by  a  cross  wall 
at  the  base,  and  then  divides  transversely  to  form  a 
stalk  cell  and  a  body  cell  (Fig.  35,  A).  The  latter 
becomes  large  and  spherical  and  its  nucleus  divides 
into  eight  by  successive  bipartitions.  These  eight  nuclei 
have  different  fates  in  different  species.  In  some  the 
cytoplasm  divides  correspondingly  and  eight  eggs  are 
formed  in  the  oogonium  (Fig.  35,  B),  but  in  others 
four  or  six  of  the  nuclei  degenerate  and  disappear 
and  the  whole  of  the  cytoplasm  forms  four  or 
only  two  eggs.  In  others,  again,  seven  of  the 
eight  nuclei  degenerate  and  only  one  egg  is 
formed  from  the  whole  of  the  protoplasm  of  the 
oogonium.1  The  oogonium  wall  has  two  layers  :  the 
outer  bursts,  setting  free  the  inner  wall  as  a  bladder 
enclosing  the  eggs  (Fig.  35,  C). 

The  male  organs  (antheridia)  are  club-shaped  cells 
(Fig.  34,  A,  a),  which  are  branches  of  a  hair  that  arises, 
like  the  oogonium,  from  a  surface  cell  of  the  con- 
ceptacle. The  nucleus  of  the  antheridial  cell  divides  by 
repeated  bipartitions  to  form  64  nuclei  (Fig.  34,  C,  «), 
and  the  cytoplasm  divides  correspondingly  so  that 
64  sperms  are  formed  (D).  Each  of  these  is  a  minute 
biflagellate  pear-shaped  cell  containing  besides  the 
nucleus  (Fig.  34,  D,  E,  n)  a  single  orange  phaeoplast. 
The  mass  of  64  sperms  is  freed  from  the 

1  This  process  is  essentially  the  same  as  the  formation  of  the  so- 
called  "  polar  bodies  "  of  animal  eggs.  In  the  animal  a  brood  of  four 
gametes  is  formed  from  the  mother  cell,  of  which  three  degenerate 
(polar  bodies),  leaving  a  single  egg.  In  Fucus  and  allied  genera  a 
brood  of  eight  gametes  is  formed,  of  which  all  are  fertile  eggs,  or  in 
some  species  four,  six  or  seven  degenerate. 


224  DIFFERENTIATION   OF   TISSUES.      FUCUS 

antheridial  hair    enclosed   in    the    inner    wall    of    the 
antheridium. 

The  bladders  containing  the  eggs  and  sperms  are 


D 


FIG.  34. — A,  branched  hair  from  conceptacle  of  Fucus  showing 
antheridia  (a)  and  "  sterile  "  cells  (st.).  B,  bladder,  formed 
by  inner  wall  of  antheridium,  bursting  and  setting  free  the  sperms 
(male  gametes).  C  and  D,  stages  in  development  of  antheridium. 
E,  free  male  gamete  with  two  flagella];  n,  nucleus ;  ph.,  phaeoplast. 

forced  through  the  pore  forming  the  mouth  of  the 
conceptacle  by  the  pressure  of  the  swollen  walls  of 
the  hairs  which  fill  up  most  of  the  space  within  its 


FERTILISATION   AND   GERMINATION   OF   ZYGOTE         225 

cavity.  They  are  expelled  from  the  conceptacle  at 
low  tide  when  the  plants  are  exposed  to  the  air,  and 
lie  in  masses  in  the  film  of  water  adhering  to  the  slimy 
surface  of  the  frond.  The  masses  of  sperm  bladders, 
where  these  are  separate  from  the  eggs,  can  be  easily 
distinguished  with  the  naked  eye  by  their  bright 
orange  colour  due  to  the  orange  phseoplasts  of  the 
sperms.  The  bladders  enclosing  the  sperms  and  eggs 
then  burst  (Figs.  34,  B,  35,  C),  setting  the  gametes  free 
in  the  film  of  water. 

The  spherical  egg  contains  much  food  material  and 
is  many  times  the  diameter  of  the  sperm.  It  secretes 
a  substance  of  unknown  nature  which  attracts  the 
sperms,  and  these,  usually  in  considerable  numbers, 
cluster  round  it  (Fig.  35,  D),  and  by  their  constant 
oscillation  frequently  set  up  a  vortex  in  which  the  egg 
rotates.  One  of  the  sperms  eventually  penetrates 
the  egg  and  the  sperm  nucleus  travels  across  the 
cytoplasm  and  fuses  with  the  egg  nucleus  (Fig.  35,  E, 
F,  G).  Though  the  sperm  nucleus  is  much  smaller  than 
that  of  the  egg,  the  amount  of  chromatin  it  contains  is 
seen  to  be  equivalent  in  bulk,  as  is  always  the  case. 

Development  of  the  Young  Plant. — The  fertilised 
egg  (zygote)  germinates  at  once.  A  cell  wall  is 
secreted,  the  zygote  lengthens,  one  end  becoming 
pointed,  and  the  nucleus  divides  by  a  cross  wall ;  the 
cell  corresponding  with  the  pointed  end  forming  the 
first  attaching  organ  or  rhizoid,1  which  sticks  to  any 
solid  substratum  with  the  help  of  its  mucilaginous 
wall.  The  upper  cell  divides  repeatedly,  and  this  part 
of  the  plant  body  elongates  into  a  club-shaped  form 
(Fig.  33,  B),  fresh  rhizoids  meanwhile  growing  out  at 
the  base.  Very  soon  an  apical  cell  is  established  on 

1  Root-like  organ,  Greek  pt'Ca,  root. 
15 


226  DIFFERENTIATION   OF  TISSUES.      FUCUS 


DIFFERENTIATION  OF  SURFACE  AND  INTERNAL  CELLS      227 

the  surface  in  the  centre  of  the  upper  end,  and  by  the 
more  rapid  growth  and  division  of  the  neighbouring 
surface  cells  the  apical  cell  shortly  becomes  sunk  in 
a  hollow.  Up  to  this  point  the  young  plant  is  nearly 
cylindical  in  form,  but  now  the  upper  part  grows  more 
quickly  in  one  longitudinal  plane  than  in  the  plane 
perpendicular  to  it,  and  thus  the  wings  of  the  frond 
are  started. 

A  marked  difference  is  soon  apparent  between  the 
growth  of  the  surface  cells  and  those  occupying  the 
centre  of  the  thallus.  The  former  remain  small, 
divide  actively  and  form  the  palisade  (surface)  layer. 
The  cells  lying  immediately  below  do  not  divide  so 
rapidly  and  form  the  cortex.  The  central  cells  are  still 
more  sluggish  and  become  passively  stretched  in  the 
longitudinal  direction  as  the  surface  increases  in 
extent  by  active  growth  and  division  of  the  outer 
cells  (Fig.  33,  A).  At  the  same  time  the  middle  layers 
of  their  walls  become  mucilaginous  and  separate  the 
bodies,  central  cells  from  one  another  laterally,  thus 
giving  rise  to  the  characteristic  structure  of  the 
medulla.1 

This  same  contrast  between  outer  and  inner  cells 
can  also  be  seen  in  other  Brown  Seaweeds  (not  at  all 
closely  allied  to  Fucus)  whose  thallus  consists  of  stout 

FIG.  35. — A,  developing  oogonia  from  surface  of  conceptacle  with 
accompanying  hairs.  B,  body  cell  of  oogonium  containing  eight 
young  eggs  (female  gametes).  C,  freeing  of  the  eight  eggs  by 
bursting  of  the  bladder  surrounding  them.  D,  single  egg  (female 
gamete)  surrounded  by  swarming  male  gametes.  E,  fertilisation. 
The  nucleus  of  a  sperm  (male  gamete)  (sp.n.)  has  penetrated  the 
egg  and  is  approaching  its  nucleus.  F,  sperm  nucleus  in  contact 
with  wall  of  egg  nucleus.  G,  sperm  nucleus  inside  egg  nucleus. 

1  The  hypha-like  chains  of  thick-walled  cells  (fibres)  which  form 
the  whole  of  the  holdfast,  the  greatest  part  of  the  medulla  of  the 
stipe,  and  also  appear  in  the  midribs  of  the  fronds,  begin  to  grow  out 
from  the  cortical  and  the  medullary  cells  as  soon  as  these  separate, 
and  the  "  hyphae  "  grow  independently  in  the  mucilaginous  matrix. 


228  DIFFERENTIATION   OF  TISSUES.      FUCUS 

cylindrical  threads  from  six  to  twenty  cells  thick.  The 
surface  cells  are  small  and  densely  filled  with  phaeo- 
plasts,  the  central  cells  are  larger  and  especially 
longer,  and  possess  many  fewer  phaeoplasts  per  unit 
volume  (Fig.  33,  C). 

We  can  only  relate  this  difference  between  surface 
and  central  cells,  which  is  quite  a  general  feature  of 
bulky  algae,  to  the  much  more  favourable  conditions 
for  active  growth  and  division  in  which  the  surface 
cells  are  placed.  They  get  more  oxygen  and  more 
dissolved  salts,  for  all  these  are  obtained  directly  from 
the  surrounding  water  ;  they  are  also  better  illumi- 
nated. Thus  it  appears  that  the  differentiation  of 
tissues  in  these  algae  depends  directly  on  the  different 
conditions  under  which  the  cells  develop.  The  dif- 
ferent functions  of  the  tissues  in  the  adult  alga  are 
determined  by  their  structure  and  position  in  relation 
to  the  source  of  food  and  oxygen  and  to  the  growth 
of  the  thallus  as  a  whole.  Since  growth  is  localised 
at  the  tips  of  the  branches,  the  medullary  cells,  as 
explained  on  p.  220,  owing  to  their  structure  and  posi- 
tion, are  the  natural  channels  of  conduction  of  the 
organic  foodstuffs  elaborated  by  the  photosynthetic 
cells. 

This  is  a  case  in  which  the  origin  of  differentiation 
in  organisms  appears  to  be  directly  due  to  differences 
in  the  conditions  in  which  different  cells  develop, 
i.e.  to  physical  and  chemical  differences  of  environ- 
ment. The  differentiation  so  initiated  is,  if  it  results 
in  a  workable  mechanism,  fixed  and  further  specialised 
in  higher  forms. 

In  the  Laminariaceae,  the  group  of  Brown  Sea- 
weeds which  have  the  longest  bodies,  including  the 
gigantic  Macrocystis  and  also  the  big  "  tangles " 


STRUCTURE   OF   FUCUS  22Q 

(Laminaria)  of  our  own  coasts,  some  of  the  medullary 
cells  are  very  highly  specialised  for  conduction  of 
organic  substances,  and  are  strikingly  like,  even  in 
small  details,  the  sieve  tubes  of  the  higher  plants, 
described  in  Chapter  XVI. 

PRACTICAL   WORK. 

Fucus. 
Vegetative  Structure 

(1)  Make  a  sketch  of  samples  of  the  thallus  of  Fucus,  showing 
the  holdfast,  the  stipe,  the  branching,  the  positions  of  the  apical 
grooves,  and  the  conceptacles.     Note  that  the  stipe  is  extremely 
tough  and  cannot  be  broken  by  pulling  with  the  hands,  whereas 
the  frond  can.     If  the  frond  is  broken  the  hair-like  fibres  may 
be  seen  projecting  from  the  broken  surfaces. 

(2)  Examine  a  cross-section  of  the  frond  under  the  low  power 
and  draw  a  diagram  x  of  the  general  plan  of  distribution  of  the 
tissues,  marking  : — 

(a)  the  palisade  (photo synthetic]  layer  on  the  surface, 

(b)  the  cortex  of  large  isodiametric  cells,  with  fewer  phaeo- 

plasts  per  unit  volume, 

(c)  the  medulla  of  elongated  cells  running  in  strand  in  various 

directions  ajid  separated  by  a  matrix  of  cell  wall  sub- 
stance. 

Note  that  the  extra  thickness  of  the  midrib  is  caused  by  the 
greater  thickness  of  the  medulla  in  that  region. 

(3)  Examine  a  cross-section  of  the  frond  under  the  high  power, 
and  make  careful  drawings  *  of  small  samples  of  the  various  tissues, 
including  the  thick-walled  fibres  ("  hyphae  "),  which  are  mainly 
localised  just  below  the  cortex  of  the  midrib,  on  the  edge  of 
the  medulla.     Note  that  the  conducting  cells  run  horizontally 
or  obliquely  in  the  wings,  longitudinally  (so  that  they  are  cut 
transversely)  in  the  midrib. 

(4)  Examine  two  longitudinal  sections  of  the   frond  (a)   cut 
through  the  midrib  at  right  angles  to  the  surface  of  the  frond, 
(b)  cut  parallel  to  the  surface  of  the  frond  through  the  centre. 
Identify  the  various  tissues  already  seen  in  transverse  section, 
and  from  a  comparison  of  the  appearance  of  the  cells  in  the 

1  See  p.  19.  *  See  p.  20. 


23O  DIFFERENTIATION   OF  TISSUES.      FUCUS 

two  views  deduce  their  shapes.  Draw  under  the  high  power 
samples  of  the  cells  which  are  not  identical  in  appearance  in 
transverse  and  longitudinal  sections.  Note  the  thin  transverse 
walls  of  the  conducting  cells. 

(5)  Examine  transverse  and  longitudinal  sections  of  the  stipe, 
noting  the  differences  between  its  structure  and  that  of  the  frond, 
especially   (a)  the  worn  surface  with  radial  splits  between  the 
surface  cells  ;    (b)  the  absence  of  a  distinct  palisade  layer  and 
the  radial  rows  of  cortical  cells  with  relatively  thin  tangential 
walls,   indicating  that  this  layer  has   been  largely   formed   by 
secondary  cell  division  parallel  to  the  surface  ;    (c)  the  mass  of 
fibres  ("  hyphae  ")  mostly  cut  transversely,  forming  the  medulla, 
interspersed  with  large  isolated  cells  (the  original  medullary  cells) . 
Make  drawings  to  illustrate  these  points. 

In  the  longitudinal  section  of  the  stipe  identify  the  tissues 
seen  in  transverse  section. 

(6)  Examine  a  longitudinal  section  through  the  apex  of  the 
frond,    showing   the   apical   cell    and   the   origin  of  the    adult 
tissues. 

Sexual  Organs. 

(7)  Examine  a  section  across  the  reproductive  region  of  the 
thallus,  first  under  the  low  power.     Note  that  the   vegetative 
tissues  have  the  same  general  characters  as  in  the  purely  vegetative 
part  of  the  frond,  but  that  the  medullary  cells  here  form  a  net- 
work, the  cells  being  cut  in  various  directions  :  this  is  the  result 
of  the  increase  in  thickness  of  this  part  of  the  frond,  the  strands 
of  cells  being  drawn  out  in  all,  not  only  in  one  direction. 

(8)  Examine  the  development  and  structure  of  the  antheridia 
and  oogonia  in  the  same  or  in  different  conceptacles,  and  draw 
as  many  stages  of  development  as  you  can  distinguish.     Note 
also  the  "  sterile  "  hairs  in  the  conceptacle. 

(9)  Examine  fresh  material  in  which  eggs  and  sperms  have 
been  liberated,  and  appear  as  little  masses  on  the  outside  of  the 
frond.     In  species  with  the  sexes  on  separate  plants  the  sperms 
can  be  distinguished  by  their  bright  orange  colour.      Mix  some 
eggs  and  sperms  in  a  drop  of  sea-water  or  in  salt  solution  of  about 
the  same  concentration.     Draw   under  the  high  power  an  egg 
and  some  sperms  and  watch  the  movements  of  the  latter.     The 
early  stages  of  fertilisation  (conjugation)  can  often  be  seen. 


CHAPTER   XIV 

THE  SIMPLEST  LAND  PLANTS  :    LIVERWORTS 
AND   MOSSES.     THE   PTERIDOPHYTA. 

HITHERTO  we  have  been  dealing  entirely  (except  in 
the  case  of  the  Fungi)  with  plants  which  live  in  water, 
and  get  the  whole  of  the  raw  materials  of  their  food 
supply  from  water  and  the  substances  dissolved  in  it. 
But  very  far  back  in  the  history  of  the  earth  some 
plants  emerged  from  the  water  and  established  them- 
selves on  the  land,  henceforward  getting  their  carbon 
dioxide  and  their  oxygen  for  respiration  direct  from 
the  air.  These  terrestrial  plants  have,  in  course  of 
time,  dominated  the  surface  of  the  earth,  and,  like 
the  terrestrial  animals,  have  developed  the  most 
complex  structures.  The  most  highly  developed  group 
are  the  seed  plants  or  flowering  plants  which  are  the 
most  completely  adapted  to  land  life.  The  history 
of  the  evolution  of  the  plant  kingdom,  beyond  the 
stage  of  the  earliest  land  plants,  is  mainly  a  history 
of  increasing  adjustment  to  terrestrial  conditions.  We 
know  nothing  of  how  plants  succeeded  in  first  emerging 
from  the  water,  nor  can  we  follow  in  detail  the 
course  of  their  subsequent  evolution.  But  by  study- 
ing the  simplest  land  plants  now  existing,  plants 
which  have  become  stabilised,  so  to  speak,  at  an  early 
stage  of  adjustment  to  terrestrial  conditions,  we  can 


232  THE   SIMPLEST  LAND   PLANTS 

get  some  idea  of  how  these  conditions  affected  the 
land  migrants,  and  of  the  adjustments  in  structure 
and  function  that  have  taken  place.  It  must  be 
clearly  understood,  however,  that  the  simplest  forms 
of  land  plants  now  existing  certainly  do  not  represent 
the  actual  stages  in  the  evolution  of  land  vegetation. 
They  represent  rather  side  lines  of  the  evolutionary 
tree  which  have  become  stabilised,  and  are  probably 
incapable  of  much  further  evolution. 

The  plants  known  as  Liverworts  and  Mosses  are 
two  groups  of  green  plants,  all  comparatively  small 
and  with  tissues  of  comparatively  simple  organisation. 
They  resemble  one  another  in  the  structure  of  their 
reproductive  organs  and  in  their  life  histories,  but 
are  distinctly  different  in  the  form  and  structure  of 
the  plant  body.  In  the  last  respect  the  Mosses  are 
decidedly  more  highly  developed  than  the  Liverworts. 
Both  represent  comparatively  low  stages  in  adapta- 
tion of  the  green  plant  to  terrestrial  life.  Most  of 
them  live  in  situations  where  the  soil  and  the  air  are 
constantly  moist,  so  that  they  are  in  little  danger  of 
losing  water  by  evaporation  so  quickly  that  they  dry 
up.  They  are  not  nearly  so  completely  protected  as 
the  seed  plants  against  this  risk  by  having  an  almost 
impermeable  waterproof  covering,  though  many  of 
them  are  protected  to  some  extent.  And  they  can 
absorb  water  over  much  or  over  the  whole  of  their 
surface,  which  the  higher  plants  cannot  do.  Some 
species  actually  live  in  places  (e.g.  rock  surfaces,  walls, 
etc.)  where  they  are  liable  to  dry  up,  and  these  can 
remain  alive,  though  dormant,  in  the  dry  condition  ; 
when  wetted  they  quickly  absorb  water  and  resume 
active  life. 

Liverworts  :    Pellia. — This    is   a   common    liverwort 


THALLUS   OF  PELLIA 


233 


growing  on  soil  among  damp  herbage,  or  in  marshes. 
Pellia  has  no  differentiation  into  distinct  stem  and 
leaves,  any  more  than  Fucus  has.  The  body  con- 
sists of  a  flat  green  thallus,  which  may  be  elongated 


rlt 


FIG.  36. — Pellia.  A,  plant  (natural  size)  growing  in  bright  light 
with  crowded  branches;  spn.,  sporogonia  of  various  ages,  the 
oldest  with  long  stalks  and  open  capsules.  B,  part  of  thallus 
on  a  larger  scale,  showing  forking  at  the  tip;  rh.,  rhizoids  arising 
from  lower  side;  a,  antheridia;  spn.,  sporogonium  with  opened 
spore  capsule  showing  bunch  of  hair-like  elaters  (el.). 

and  band-shaped,  or  may  (especially  in  bright  light) 
grow  slowly  in  length  and  branch  freely  so  that  the 
plant  is  tufted  (Fig.  36).  The  margins  of  the  thallus 
(wings)  are  often  "  crisped  "  because  they  have  grown 
quicker  than  the  thicker  central  part,  or  midrib.  The 
latter  bears  on  its  under  surface  brown  rhizoids 
(Fig.  36,  B,  rh.},  which  enter  the  soil  and  absorb  from 
it  water  and  dissolved  salts. 

A  thin  cross-section  of  the  thallus  shows  that  all 
the  cells  are  living  and  may  contain  chloroplasts, 
though  these  are  mainly  concentrated  in  the  upper 
(sometimes  also  in  the  lower)  surface  layer.  This 
represents  an  incipient  differentiation  of  photosynthetic 


234  THE   SIMPLEST   LAND   PLANTS 

tissue,  but  there  is  little  differentiation  in  the  structure 
of  the  cells.  In  the  surface  cells  starch  grains  may  be 
detected  enclosed  in  the  chloroplasts  if  the  plant  has 
been  exposed  to  fairly  bright  light ;  in  the  central 
cells  there  are  large  starch  grains  which  have  been 
formed  by  chloroplasts,  out  of  which  they  have  burst, 
and  which  may  sometimes  be  still  detected  as  green 
smears  on  the  surfaces  of  the  grains  (cf.  Pellionia, 
p.  126).  The  rhizoids  are  seen  to  be  tubular  outgrowths 
of  the  cells  on  the  lower  surface  of  the  midrib.  The 
midrib  passes  gradually  into  the  wings,  which  at  their 
outer  edges  are  only  one  cell  thick.  Pellia,  like  Fucus, 
grows  by  division  of  apical  cells  at  the  tips  of  the 
branches  of  the  thallus.  The  cells  cut  off  from  these 
divide  further  to  form  the  whole  of  the  tissue  of  the 
thallus. 

Reproduction  and  Life  History. — Pellia,  like  Fucus, 
and  also  like  all  the  higher  plants,  is  reproduced  by 
means  of  sexually  differentiated  gametes  ;  and  as  in 
Fucus  these  are  formed  in  sexual  organs  arising  from 
surface  cells  of  the  thallus.  There  is  an  important 
difference,  however  :  in  Pellia  (as  in  all  the  Liverworts 
and  Mosses  and  in  the  higher  group  called  Pteridophyta) 
the  sexual  organ,  both  male  and  female,  is  covered  in 
the  mature  state  by  a  wall  consisting  of  a  layer  of 
cells  (Fig.  37,  A,  C,  w),  instead  of  by  a  cell  wall  only 
as  in  Fucus.  This  we  may  perhaps  relate  to  the 
greater  protection  from  evaporation  required  by  sub- 
aerial  life.  The  antheridia  (Fig.  37,  A)  are  spherical 
structures  formed  on  the  upper  surface  of  the  thallus, 
each  in  a  little  cavity  due  to  the  arrest  of  cell  divi- 
sion in  the  thallus  tissue  just  below  the  spot  where 
the  antheridium  is  formed  and  the  growing  out  of 
the  thallus  cells  to  roof  in  the  antheridium  on  each 


SEXUAL   ORGANS   OF  PELLIA 


235 


side.  The  sperms,  very  many  of  which  are  produced 
in  each  antheridium,  are  not  unlike  those  of  Volvox 
in  shape,  and  like  them  have  two  flagella,  but  there 
is  no  trace  of  a  chloroplast,  and  the  long,  narrow, 


FIG.  37. — Pellia.  A,  part  of  cross-section  of  thallus,  showing  ripe 
antheridium;  w,  wall  of  antheridium;  spe.,  sperms  (male)  gametes 
coiled  up.  X  80.  B,  single  male  gamete  (after  Guignard, 
X  122).  C,  four  archegonia  in  various  stages  of  development.  On 
the  left  the  archegonium  is  nearly  ripe  ;  w,  wall  of  venter  con- 
taining egg ;  n,  neck,  which  will  shortly  open  at  the  top.  D,  group 
of  multicellular  spores  (spo.)  with  spirally  thickened  elaters  (el.). 


236  THE   SIMPLEST  LAND   PLANTS 

spirally  coiled  body  of  the  sperm  consists  almost 
wholly  of  nucleus,  with  a  little  colourless  cytoplasm 
at  the  front  end  to  which  the  flagella  are  attached 
(Fig.  37,  B).  The  sperm  cell  is  here  reduced  to  its 
lowest  limits — the  paternal  nucleus  and  the  flagella, 
which  are  the  means  of  carrying  the  nucleus  to  the  egg. 

The  female  organs  are  formed  in  groups,  overhung 
by  a  membrane  which  grows  out  from  the  thallus 
behind  them.  They  are  flask-shaped,  and  in  each 
there  is  a  single  egg  contained  in  the  body  (venter  x) 
of  the  flask,  which  it  practically  fills  (Fig.  37,  C).  The 
neck  of  the  flask  is  composed  of  a  single  layer  of.  cells 
continuous  with  those  forming  the  wall  of  the  venter. 
There  is  open  communication  from  the  egg  to  the 
exterior  through  the  channel  of  the  neck,  which  is 
filled  with  mucilage.  This  flask-shaped  type  of  female 
organ,  which  is  characteristic  of  the  Liverworts  and 
Mosses  and  also  of  the  lower  vascular  plants  (Pteri- 
dophyta),  is  called  an  archegonium. 

When  a  film  of  water  (rain  or  dew)  is  present  on 
the  surface  of  the  thallus  the  ripe  antheridium  opens 
by  the  swelling  and  bursting  of  its  wall  and  thus  sets 
free  the  sperms,  which  are  attracted  to  the  mouth  of 
the  archegonium  by  the  secretion  of  a  substance,  prob- 
ably cane  sugar,  produced  by  the  latter  and  diffusing 
out  into  the  water.  On  reaching  the  mouth  the 
sperms  become  entangled  in  the  mucilage,  wriggle 
down  the  neck  and  one  of  them  fuses  with  the  egg, 
which  then  becomes  a  zygote  and  secretes  a  wall. 

The  egg  of  Pellia  (Fig.  37,  C)  is  much  smaller  than 
that  of  Fucus,  and  this  is  related  to  the  fact  that  it  is 
not  cast  out  from  the  thallus,  and  does  not,  therefore, 
have  to  depend  on  its  own  resources  during  germi- 

'  Belly. 


SPOROGONIUM   OF  PELLIA  237 

nation.  The  egg  of  Pellia  is  retained,  after  fertili- 
sation, in  the  female  organ,  and  germinates  in  situ, 
so  that  the  embryo  is  protected  by  and  can  draw 
food  from  the  thallus.  This  is  the  first  example  we 
have  met  with  of  the  protection  and  feeding  of  the 
embryo  by  the  mother  organism,  a  condition  which  is 
of  great  importance,  and  is  carried  much  further  in 
the  higher  organisms,  both  plants  and  animals. 

Sporogonium. — The  embryo  of  Pellia,  and  indeed 
of  all  the  Mosses  and  Liverworts,  does  not  develop  into 
a  completely  independent  plant,  but  into  a  spore-pro- 
ducing structure  called  a  Sporogonium  (Fig.  36,  spn.}, 
which  remains  attached  to  the  thallus;  and  the  spores 
produced  by  this  germinate  to  produce  new  Pellia  plants. 

The  mature  Sporogonium  produced  from  the  fertilised 
egg  consists  of  (i)  a  foot  embedded  in  and  drawing 
organic  food  from  the  thallus,  (2)  a  long  stalk,  and 
(3)  a  spherical  head  or  spore  capsule  which  is  at  first 
green.  The  stalk  remains  short  for  some  two  or  three 
months  in  the  spring  while  the  spores  develop,  but 
when  these  are  nearly  ripe  it  quickly  lengthens 
(Fig.  36,  A),  and  in  two  or  three  days  attains  a  length 
of  2  to  3  inches.  The  spore  capsule  is  now  dark,  almost 
black,  and  in  dry  air  it  opens,  four  splits  running  down 
from  the  centre  of  its  upper  surface  to  the  base  where 
it  joins  the  stalk.  The  four  strips  of  wall  thus 
separated  fold  back  as  four  separate  flaps,  which  form 
a  cross-shaped  structure  standing  out  from  the  top 
of  the  stalk  and  exposing  the  mass  of  spores  within 
(Fig.  36,  A).  If  the  opened  capsule  be  examined  with 
a  hand  lens  there  can  be  seen,  interspersed  with  the 
dust-like  spores,  a  number  of  delicate  threads,  the 
so-called  elaters  (Fig.  36,  B,  el.}.  These  are  long  cells 
with  spirally  wound  thickenings  inside  their  walls 


238  THE  SIMPLEST  LAND  PLANTS 

(Fig.  37,  D,  el.).  A  large  bunch  of  them  arises  from 
the  base  of  the  capsule.  The  elaters  are  very  hygro- 
scopic, i.e.  very  sensitive  to  small  changes  in  the 
amount  of  water  vapour  in  the  air,  and  as  they  take 
up  or  lose  water  from  the  air  they  twist  about,  dis- 
turbing and  scattering  the  mass  of  spores,  which  float 
off  into  the  air. 

The  spores  are  large  and  multicellular  (Fig.  37,  D, 
spo.),  i.e.  the  original  spore  cell  divides  into  several 
cells  on  ripening,  and  these  contain  chloroplasts.  The 
spores  do  not  form  a  resting  stage  in  the  life  history, 
they  die  unless  they  quickly  fall  upon  damp  soil  where 
they  can  germinate.  One  of  the  cells  grows  out  and 
gives  rise  to  the  apical  cell  of  the  new  Pellia  plant, 
and  the  first  rhizoids  grow  from  the  ends  of  the  spore. 

Besides  the  thalloid  forms,  of  which  Pellia  is  one, 
there  are  many  Liverworts  whose  shoots  have  distinct 
stems  and  leaves,  the  leaf  being  a  thin  membranous 
structure  consisting  of  a  single  layer  of  cells  containing 
chloroplasts.  The  cells  of  the  stem  are  longer  and 
often  thick-walled,  but  there  is  very  little  tissue 
differentiation.  These  leafy  Liverworts  live  largely  on 
rocks  or  tree-trunks  in  wet  climates,  and  absorb  water 
through  the  whole  surface  of  the  plant,  especially 
the  leaves.  They  are  attached  by  rhizoids  to  the 
substratum  on  which  they  grow,  but  the  rhizoids  are 
of  no  importance  in  the  absorption  of  water,  as  those 
of  Pellia  are. 

Mosses. — The  Mosses  always  have  their  shoots  differ 
entiated  into  stem  and  leaf,  and  most  of  them  have 
a  certain  amount  of  tissue  differentiation,  considerably 
more  than  the  Liverworts.  They  are  on  the  whole 
larger  plants  than  the  Liverworts,  and  depend  more 
upon  absorption  of  water  from  the  soil,  though  a 


MOSSES  239 

number  of  species  of  Mosses  are  small  plants  growing 
on  rocks  and  tree-trunks  which  can  survive  desiccation. 

The  rhizoids  are  branched  multicellular  cell  threads 
which  arise  from  the  base  of  the  stem  and  enter  the 
soil,  from  which  they  absorb  water.  The  stem  has  a 
central  strand  of  long  narrow  thin-walled  cells  from 
which  the  protoplasm  has  disappeared,  and  these 
form  a  water  channel  from  the  absorbing  region 
(rhizoids)  to  the  evaporating  region  (leaves).  The 
cells  of  the  outer  layers  of  the  stem  (cortex)  have 
thick  brown  walls.  The  leaves  ordinarily  consist  of  a 
single  layer  of  cells  containing  chloroplasts,  but  there 
is  often  a  midrib,  several  cells  thick,  consisting  of 
several  cell-layers  and  possessing  thin-walled  water- 
conducting  cells  like  those  of  the  centre  of  the  stem. 
The  shoot  grows  by  the  division  of  a  single  apical  cell 
at  its  tip,  as  in  the  Liverworts  and  in  Fucus.  Each 
segment  cut  off  from  this  apical  cell  gives  rise  by 
division  to  the  tissue  of  a  single  leaf  and  to  the  segment 
of  the  stem  which  bears  it. 

The  Moss  plant  is  a  stage  further  on  than  the 
Liverwort  in  adaptation  to  land  life.  Though  they 
can  absorb  water  through  their  leaves,  many  Mosses 
have  a  regular  water  current  from  rhizoids  to  leaves. 
The  existence  of  a  definite  water-conducting  tissue 
corresponds  with  this  localisation  of  absorption  in  one 
part  of  the  body  (rhizoids)  and  of  evaporation  in  another 
(leaves),  which  is  the  mark  of  a  terrestrial  plant,  and 
which  is  absent  in  Fucus  and  almost  absent  in  Pellia. 
This  feature  is,  as  we  shall  see,  carried  to  a  much 
higher  level  of  development  in  the  Vascular  Plants. 

Sexual  Organs  and  Sporogonium  of  Mosses.— The 
sexual  organs  and  gametes  of  the  Mosses  have  a 
general  resemblance  to  those  of  Liverworts,  though 


240  THE   SIMPLEST  LAND   PLANTS 

they  differ  in  certain  details  of  structure.  The  sexual 
organs,  accompanied  by  hairs  and  surrounded  by 
leaves,  which  are  sometimes  coloured  yellow  or  red, 
are  borne  at  the  tips  of  upright  shoots  that  have 
stopped  growing. 

The  fertilised  egg  cell,  like  that  of  the  Liverwort, 
germinates  in  situ  within  the  archegonium,  and 
the  embryo  grows  into  a  sporogonium  which,  as  in  the 
Liverwort,  remains  attached  to  the  parent  plant.  The 
Moss  sporogonium,  however,  is  of  much  more  compli- 
cated structure  than  that  of  the  Liverwort.  It  has  a 
foot,  stalk  and  spore  capsule,  but  the  stalk  elongates 
much  earlier  and  possesses  a  central  water-conducting 
strand  like  that  of  the  leafy  stem.  Surrounding  this 
there  is  sometimes  a  layer  of  elongated  living  cells 
which  conduct  organic  food  to  the  developing  spore 
capsule.  These  are  comparable  in  function  with  the 
medullary  cells  of  Fucus.  The  spore  capsule  itself  is 
quite  an  elaborate  structure.  Its  wall  consists  of 
several  layers  of  cells  containing  chloroplasts  and  is 
covered  by  an  epidermis  with  stomata,  possessing  in 
fact  a  structure  like  that  of  the  leaf  of  a  higher  plant. 
Thus  the  sporogonium,  which,  during  development,  is 
supplied  with  organic  food  by  the  leafy  parent  plant,  is 
able  when  nearly  mature  to  make  some  of  its  own  food 
from  the  carbon  dioxide  of  the  air  and  the  water  and 
salts  brought  up  from  the  leafy  parent  plant  through 
the  foot  and  stalk.  When  the  spores  are  ripe  a  lid 
is  detached  from  the  top  of  the  capsule,  and  distri- 
bution of  the  spores  is  often  assisted  by  the  move- 
ments of  hygroscopic  teeth  set  round  the  edge  of  the 
opening,  which  thus  perform  the  same  function  as  the 
elaters  of  the  Liverwort  capsule.  The  spores  germi- 
nate on  damp  soil  to  form  an  alga-like  growth  (proto- 


VASCULAR   PLANTS  24! 

nemo)  of  branching  green  cell  threads,  which  spreads 
and  often  persists  for  a  long  time  on  damp  soil. 
From  the  protonema  leafy  moss  plants  arise  by  budding. 

Vegetative  Reproduction.— Both  Mosses  and  Liver- 
worts spread  largely  by  vegetative  reproduction.  The 
thalli  of  Liverworts  branch  freely  as  they  creep  on 
the  soil,  and  the  older  parts,  from  which  the  branches 
have  arisen,  gradually  die  off,  so  that  the  branches, 
which  have  sent  out  rhizoids  from  their  under  surfaces, 
become  detached  independent  plants.  In  the  Mosses 
branching  often  takes  place  towards  the  base  of  the 
shoot,  and  the  lateral  branches,  creeping  on  or  in  the 
soil,  become  attached  by  rhizoids,  their  tips  growing 
up  into  the  air.  The  decay  of  the  older  part  of  the 
shoot  makes  the  new  shoots  independent  plants. 
Very  many  Mosses  branch  at  the  base  in  this  way, 
and  the  aerial  shoots  so  produced  grow  up  in  close 
neighbourhood,  their  tips  forming  the  surface  of  the 
characteristic  moss  tufts  or  cushions. 

Vascular  Plants  (Pteridophytes  and  Seed  Plants). — 
The  plants  above  the  Mosses  in  the  scale  of  adaptation 
to  terrestrial  life  are  marked  by  several  great  steps  in 
advance.  First  they  have  true  roots  instead  of  rhizoids. 
These  are  branches  of  the  plant  body  many  cells  thick 
which  enter  the  soil,  branch  and  form  very  efficient 
organs  for  fixing  the  plant  and  absorbing  water  and 
salts  from  the  soil.  The  branches  of  the  root  are 
comparable  with  the  branches  of  the  shoot,  and  like 
them  consist  of  complicated  tissue  structures,  but 
they  differ  from  them  in  several  important  respects. 
We  shall  have  to  consider  these  differences  in  detail 
in  later  chapters.  Here  we  need  only  note  that  roots 
are  colourless,  and  that  they  often  bear  root  hairs, 
which  are  tubular  outgrowths  of  the  surface  cells 
16 


242  THE   SIMPLEST  LAND   PLANTS 

like  the  rhizoids  of  Pellia,  increasing  the  absorptive 
surface. 

Secondly,  there  is  a  highly  differentiated  double 
conducting  tissue  system — the  vascular  system — run- 
ning throughout  the  plant,  and  consisting  of  a  water- 
conducting  and  an  organic  food-conducting  portion. 
We  have  already  seen  that  something  of  the  kind, 
though  not  very  highly  differentiated,  exists  in  the 
stalk  of  the  sporogonium  in  Mosses.  This  double 
conducting  system  is  a  necessity  in  bulky  plants  in 
which  the  regions  of  water  absorption,  evaporation, 
photosynthesis  and  growth  are  all  localised. 

Thirdly,  the  photosynthetic  organs,  the  foliage  leaves, 
are  typically  plates  of  tissue  several  cells  thick,  in 
which  the  photosynthetic  tissue  (mesophyll)  is  inter- 
penetrated by  intercellular  air  spaces  and  is  covered 
externally— in  common  with  the  whole  shoot — by  a  layer 
of  cells  (epidermis)  without  chloroplasts,  whose  outer 
walls,  in  contact  with  the  air,  develop  a  waterproof 
layer,  the  cuticle.  The  epidermis  and  cuticle  are  pierced 
by  pores  (stomata),  each  surrounded  by  a  pair  of  special 
cells  (guard  cells]  containing  chloroplasts  which  regulate 
the  size  of  the  pore  (Figs.  50,  51). 

Most  of  these  characters  are  foreshadowed  in  the 
Mosses.  In  the  sporogonial  wall,  for  instance,  we 
met  with  the  essential  characters  of  the  mesophyll, 
epidermis  and  stomata  of  the  foliage  leaf ;  and  a 
cuticle  is  developed  more  or  less  strongly  both  on  the 
outer  surface  of  the  sporogonium  and  elsewhere.  But 
the  cuticle  is  neither  so  universal  nor  (usually)  so 
strongly  developed  in  Mosses  and  Liverworts  as  in 
the  Vascular  Plants.  Again,  we  saw  that  rudiments 
of  the  double  conducting  system  are  found  in  the 
stalk  of  the  sporogonium,  which  is  in  all  respects  the 


THE    PTERIDOPHYTA  243 

structure  most  highly  adapted  to  subaerial  life  below 
the  level  of  the  Vascular  Plants  themselves.  And 
finally,  in  one  of  the  highest  families  of  Mosses  we 
have  underground  shoots  which  have  many  of  the 
characters  of  true  roots.  But  in  their  totality  the 
features  described  above  are  characteristic  of  and — 
apart  from  a  few  aberrant  forms  adapted  to  special 
conditions  of  life — are  universally  found  only  in  the 
Vascular  Plants,  which  include  the  Pteridophytes  and 
the  Seed  Plants. 

The  Pteridophyta  (Ferns,  Clubmosses  and  Horse- 
tails).— Important  as  these  plants  are  to  the  botanist, 
it  is  beyond  the  scope  of  this  book  to  devote  to  them 
any  special  study.  All  the  vegetative  characters  of 
the  Vascular  Plants  can  for  our  purposes  be  more 
conveniently  considered  in  connexion  with  the  Seed 
Plants.  But  some  knowledge  of  the  outline  of  the 
life  histories  of  Pteridophytes  is  essential  to  under- 
standing the  later  phases  of  adaptation  to  terrestrial 
life  which  are  characteristic  of  the  Seed  Plants. 

In  the  Mosses  and  Liverworts  we  have  two  kinds  of 
reproduction  which  regularly  alternate  with  one  another 
in  the  life  history — the  sexual  process  of  fertilisation, 
which  can  only  be  carried  out  in  water  owing  to  the 
fact  that  the  sperms  are  motile  swimming  cells,  and 
the  process  of  spore  formation  and  dispersal,  which 
takes  place  in  air.  This  emphasises  the  incomplete- 
ness of  adjustment  of  these  plants  to  terrestrial  (sub- 
aerial)  life.  In  the  Pteridophyta,  which  include  three 
or  more  divergent  stocks  or  phyla  of  plants  originating 
so  far  back  in  early  geological  time  (certainly  before 
the  Devonian  rocks  were  laid  down)  that  we  know 
nothing  of  their  actual  origin,  we  still  find  these  two 
alternating  types  of  reproduction,  in  spite  of  the 


244  THE    PTERIDORHYTA 

development  ot  the  structural  characters  of  the  vege- 
tative body  described  in  the  last  section,  which  adapt 
them  for  terrestrial  life. 

In  the  Ferns  we  find  a  thalloid  structure  called  the 
prothallus  (Fig.  38,  A),  resembling  in  a  general  way  the 
thallus  of  Pellia,  which  can  only  live  in  damp  places, 
which,  like  Pellia,  forms  rhizoids  (rh.)  on  its  lower 
surface,  and  which  bears  sexual  organs  (Fig.  38,  A,  an, 
a,  B  and  D)  very  much  like  those  of  Pellia  (but  only 
on  its  lower  surface).  The  sperms  are  formed  and 
liberated  in  much  the  same  way,  though  they  differ 
in  having  numerous  flagella  (Fig.  38,  C)  instead  of 
only  two,  and  fertilisation  takes  place  in  a  film  of 
water  on  the  surface  of  the  prothallus.  The  fertilised 
egg  (zygote)  germinates  in  situ,  producing  an  embryo 
within  the  archegonium  wall.  The  embryo  soon  sends 
a  sucker  (the  foot]  into  the  tissue  of  the  prothallus 
(Fig.  38,  E).  But  now  comes  a  wide  divergence.  Instead 
of  developing  into  a  sporogonium  which  remains 
attached  to  the  thallus  during  its  whole  life,  the 
embryo  of  the  Fern  soon  develops  a  root  which  pene- 
trates the  soil,  and  a  leaf  which  rises  above  the  soil  and 
begins  to  carry  on  photosynthesis  (Fig.  38,  F,  r  and  /). 
The  development  of  the  stem  is  slow,  but  a  second 
larger  leaf  and  a  second  larger  root  are  shortly  pro- 
duced. During  this  time  the  young  plant  has  been 
drawing  food  from  the  prothallus,  but  after  a  while  it 
becomes  quite  independent  (the  prothallus  ultimately 
dying  off),  and  grows  constantly  bigger,  successive 

FIG.  38. — Life  history  of  Fern.  A,  prothallus  from  below  ;  rh., 
rhizoids;  an.,  antheridia;  a,  archegonia.  B,  antheridium  con- 
taining coiled  sperms.  C,  single  sperm  with  numerous  flagella. 
D,  archegonium;  e,  egg;  n,  neck.  E,  section  through  embryo 
drawn  in  outline,  showing  prothallus,  foot,  beginnings  of  first 
root,  first  leaf,  and  position  of  growing  point  of  stem,  not  yet 
developed  ;  rh.,  rhizoids  of  prothallus.  F,  young  plant  still 


LIFE    HISTORY   OF   FERN 


245 


attached  to  prothallus  with  well-developed  first  leaf  and  root  ; 
st.,  stem  (still  small).  G,  pinnule  of  fern  leaf  from  below  showing 
seven  sori  (so.)  and  ripe  sporangia  (spg.)  protruding  from  below 
membrane  of  sorus.  H,  single  sporangium  opening  to  set  free 
spores  (sp.).  I,  spore  (sp.)  germinated  to  form  prothallus  ;  rh., 
rhizoids;  a,  apical  cell. 


246  THE   PTERIDOPHYTA 

leaves  becoming  larger  and  larger,  and  more  and  more 
complex  in  structure  till  they  approximate  to  the  size 
and  structure  of  the  big  compound  leaves  characteristic 
of  most  kinds  of  adult  ferns.  The  internal  structure 
of  the  body  also  increases  in  complexity,  developing 
the  characteristic  double  vascular  system. 

Our  common  ferns  are  mostly  very  much  larger 
than  Liverworts  or  Mosses,  and  the  Tree  Ferns  of 
the  tropics  are  often  20  or  30  feet  high.  This  great 
increase  in  stature  is  dependent  on  the  development 
of  the  vegetative  characters  described  in  the  last 
section,  which  provide  a  greatly  improved  equipment 
for  terrestrial  life,  and  enable  the  fern  to  grow  indefi- 
nitely, constantly  putting  out  new  leaves  and  new 
roots.  The  older  parts  of  the  plant  body  tend  to 
die  off. 

The  adult  fern  leaves  (fronds)  form  spores  similar  to 
the  spores  of  Mosses  and  Liverworts.  These  are  pro- 
duced on  the  under  sides  of  the  fronds  in  little  bag-like 
structures  (sporangia),  which  are  formed  in  groups 
(son)  usually  covered  with  a  membrane  (Fig.  38,  G). 
The  sporangia  can  just  be  seen  with  the  naked  eye. 
When  the  spores  are  ripe  the  walls  of  the  sporangia 
split  open  and  set  the  spores  free  to  float  in  the  air 
(Fig.  38,  H).  On  damp  soil  the  spore  germinates  to 
form  the  prothallus,  which  bears  the  sexual  organs. 

Here  it  may  be  useful  to  summarise  the  life  histories 
of  the  Liverwort,  the  Moss  and  the  Fern  in  the  form 
of  a  table  (see  opposite  page). 

The  plant  bearing  sexual  organs  is  called  the 
gametophyte,  the  spore-bearing  generation  (free  living 
only  in  the  case  of  the  Ferns  and  other  Pteri- 
dophytes)  the  sporophyte.  In  the  Moss  the  game- 
tophyte is  more  highly  adapted  to  terrestrial  life  than 


LIFE    HISTORIES    OF    LIVERWORT,    MOSS    AND    FERN      247 


8  H«     •? 

E  111      I 

&       x  h 


11 


•a    I 
§1^ 


'•«  I 


Is! 

S  °  S 

!  2 


s  e 


III 

-  bo  E  o 

O      IH      di 


II-     *  s 


248  THE   PTERIDOPHYTA 

in  the  other  two,  and  the  sporogonium  of  the  Moss 
is  also  much  more  highly  fitted  for  life  in  com- 
paratively dry  air.  But  it  is  only  in  the  Fern  that 
the  spore-bearing  generation  is  free  living,  completely 
adapted  to  terrestrial  life,  while  the  prothallus  is  on 
the  same  level  of  development  as  the  thallus  of  Pellia, 
and  like  it  must  have  damp  soil  on  which  to  grow 
and  a  film  of  liquid  water  in  which  the  sperms  can 
swim  for  fertilisation.  The  Fern,  therefore,  though  a 
terrestrial  plant,  is  still  tied,  so  far  as  its  sexual  repro- 
duction is  concerned,  to  semiaquatic  conditions,  un- 
doubtedly a  heritage  from  the  aquatic  algae  from  which 
it  is  .descended. 

The  majority  of  Ferns  live  in  damp  shady  places, 
where  they  are  more  likely  to  find  suitable  conditions 
for  the  production  of  prothalli  from  their  spores  and 
the  occasional  free  water  necessary  for  the  liberation 
and  swimming  of  the  sperms.  But  some  depend  for 
their  propagation  almost  entirely  on  vegetative  repro- 
duction, and  these  can  live  in  comparatively  dry 
habitats.  The  common  bracken  fern  (Pteridium  aqui- 
linum)  is  a  good  example.  It  flourishes  on  rather 
dry  sandy  soil,  its  underground  stems  spreading  far 
and  wide  and  continually  sending  up  new  fronds, 
while  the  older  parts  gradually  die  off.  It  can  and 
does  produce  prothalli  from  its  spores  when  they  fall 
in  a  suitable  damp  spot,  but  this  is  in  many  localities 
a  rare  possibility,  and  the  great  bulk  of  this  vigorous, 
aggressive,  rapidly  spreading  plant  is  produced  vege- 
tatively. 

The  Small-Leaved  Cone-Bearing  Pteridophytes. — The 
Clubmosses  (Lycopods)  and  Horsetails  (Equiseta)  differ 
widely  from  the  Ferns  in  general  appearance,  though 
the  broad  outlines  of  their  life  history  aie  essentially 


HORSETAILS  AND  LYCOPODS.   HETEROSPORY   249 

the   same.      Instead   of  large  compound   fronds  they 
have    small    scale-like    leaves,   and  in    the    Horsetails 
these  are  so  small  that  the  great  bulk  of  the  photo- 
synthesis is  done  by  the  surface  tissues  of  the  stem. 
Another  point  in  which  they  differ  from  the  Ferns  is 
that  their  sporangia  are  borne  in  cones,  i.e.  in  con- 
nexion with  small  crowded  leaves,  mostly  differing  more 
or  less  from  foliage  leaves,  at  the  ends  of  some  of  the 
shoots  (Fig.  39,  A).     The  prothalli  also  are  unlike  those 
of  Ferns,  being  cylindrical  or  thin,  branched  structures 
instead  of  flat.     In  many  of  the  Lycopods  the  pro- 
thalli are  not  green,  but  live  saprophytically  in  humus 
or   humous   soil.     These   prothalli   grow   very   slowly, 
and  the  sporophytes  depend  very  largely  on  vegetative 
reproduction.     The   British   species   of   Lycopods   live 
mostly  on  the  hills  of  the  north  and  west  where  the 
soil  is  humous,  i.e.  contains  a  large  amount  of  plant 
debris,  which  decays  very  slowly  indeed,  owing  to  the 
cool    damp   climate.     The    Horsetails    mostly    live    in 
marshes,  by  pond  sides  and  in  damp  places  generally, 
but  one  species,  the  Field  Horsetail  (Equisetum  arvense), 
grows  very  commonly  on  hedgebanks  and  the  edges  of 
fields,  though  on  stiff,  moisture-retaining  soils.     It  is 
the  only  British  Pteridophyte  occurring  in  such  situ- 
ations and  holding  its  own  with  the  dominant  seed 
plants— the  common  hedgerow  plants.     Like  the  other 
Horsetails,  it  has  long  underground  stems  which  spread 
below  the  soil  and  send  up  the  green  aerial  shoots  at 
intervals. 

Heterospory. — In  the  great  majority  of  existing 
Pteridophytes  the  spores  are  all  alike,  but  in  a 
few  two  kinds  of  spores  are  produced,  large  mega- 
spores,  much  larger  than  ordinary  spores,  only  one  or 
four  being  produced  from  a  single  sporangium,  and 


250  THE   PTERIDOPHYTA 

small  microspores,  about  the  same  size  as  ordinary 
spores  (Fig.  39,  C).  These  are  formed  in  separate 
sporangia  (Fig.  39,  B).  In  these  heterosporous  species  the 
ordinary  kind  of  free-living  prothallus,  bearing  both 
male  and  female  sexual  organs,  is  not  produced.  The 
megaspore  on  germination  produces  a  small  mass  of 
cells  at  its  apex,  the  bottom  of  the  spore  being  some- 
times afterwards  filled  with  cells  (Fig.  39,  G),  and  on 
this  small  prothallus  one  or  more  archegonia  are 
formed.  The  microspore  produces  a  prothallus  of  even 
fewer  cells,  in  most  cases  inside  the  microspore  wall 
(Fig.  39,  D),  and  with  the  exception  of  one  or  two,  the 
whole  of  these  form  a  single  antheridium  (Fig.  39,  D). 
The  spore  absorbs  water  and  the  wall  bursts,  setting 
free  the  sperms  (E,  F).  Fertilisation  takes  place  as  in 
the  ordinary  (homosporous)  Pteridophytes.  The  ferti- 
lised egg  obtains  the  food  which  enables  it  to  grow 
into  a  self-supporting  plant  mainly  or  entirely  from 
the  organic  food  supplies  stored  in  the  megaspore  (G). 
The  far-reaching  reduction  of  the  prothallus  and  the 
suppression  of  its  powers  of  independent  growth  and 
nutrition  that  we  see  in  these  heterosporous  Pteri- 
dophytes involves  the  direct  dependence  of  the  young 
sporophyte  produced  from  the  fertilised  egg  not  only  on 
the  gametophyte  but  on  the  spore  (megaspore)  which 
produced  it,  instead  of  on  a  free-living  sexual  genera- 
tion, as  in  the  homosporous' Pteridophyte.  This  opens 
the  door,  so  to  speak,  to  a  further  adaptation  to  land 
life,  in  which  the  megaspore  is  retained  in  the 
sporangium,  and  the  sexual  generation  need  no  longer 

FIG.  39. — Selaginella,  a  heterosporous  cone-bearing  Pteridophyte. 
A,  end  of  leafy  branch,  with  three  cones  (natural  size).  B,  portion 
of  stem  of  cone  with  megasporangium  (left)  opened  showing 
megaspores,  and  (right)  microsporangium  setting  free  microspores. 
C,  microspore  (mi.)  and  megaspore  (me.)  showing  thick  wall 


HETEROSPORY 


251 


and  cell  inside  (pr.}  from  which  the  prothallus  will  be  formed.  D, 
microspore  germinated  to  form  antheridium  which  completely 
nils  the  spore  cavity.  E,  microspore  opened  with  sperms  em- 
bedded in  mucilage  formed  from  breakdown  of  walls  in  water. 
F,  two  free  flagellated  sperms.  G,  megaspore  opened  and 
germinated  to  form  prothallus  with  three  archegonia  (ar.),  the 
eggs  in  two  of  which  have  developed  into  embryos  at  the  expense 
of  the  prothallial  tissue,  while  the  third  (art)  is  unfertilised 
and  still  contains  the  egg. 


252  THE    PTERIDOPHYTA 

be  produced  under  the  conditions  of  moist  soil  and 
air  where  it  can  live  as  a  semiaquatic  and  find  the 
free  water  necessary  to  the  process  of  fertilisation  by 
motile  free  swimming  male  gametes.  The  freedom 
from  this  necessity  is  not  however  attained  by  the 
existing  heterosporous  Pteridophytes,  in  which  the 
microspores  still  germinate  only  on  damp  soil  and 
the  sperms  still  have  to  swim  to  the  archegonia  in  a 
water  film.  The  necessary  step  to  complete  freedom 
from  the  semiaquatic  habitat  is  the  retention  of  the 
megaspore  in  the  megasporangium,  where  it  may 
germinate  and  produce  the  eggs  in  a  sheltered  position, 
withdrawn  from  the  danger  of  desiccation,  and  the 
bringing  of  the  microspores  to  the  megasporangium 
or  its  immediate  neighbourhood,  so  that  the  male 
gametes  produced  from  them  may  reach  the  egg 
without  the  presence  of  external  water.  This  is  the 
step  which,  as  we  shall  see  in  the  sequel,  has  been 
taken  by  the  Seed  Plants. 

PRACTICAL  WORK. 

(1)  Examine  plants  of  Pellia,  if  possible  growing  plants.     Sketch 
the  form  of  the  band-shaped  or  crisped  green  branching  thallus, 
with  thick  midrib  attached  to  the  soil  by  rhizoids  and  passing 
gradually  into  the  thinner  wings  at  the  edges. 

(2)  Examine  a  transverse  section  of  the  fresh  thallus,  showing 
the  uniform   thin- walled  tissue.     Note  that  the  chloroplasts  are 
mostly  concentrated  in  the  upper  (sometimes  also  in  the  lower) 
surface  layer  of  cells.     In  these  small  starch  grains  can  be  seen 
if  the  plant  has  been  well  illuminated.     The  central  cells  contain 
large  starch  grains  on  the  surface  of  which  can  sometimes  be 
seen  the  remains  of  a  chloroplast.     From  the  lower  surface  of 
the  midrib  rhizoids  arise,  each  as  a  tubular  branch  of  a  single 
cell. 

Draw  under  the  high  power  samples  of  the  cells  6f  the  upper 
and  lower  surfaces  with  rhizoids  and  of  the  central  cells. 

3)  Place  some  of  the  contents  of  the  ripe  spore  capsule  of 
Pellia  in  a  drop  of  dilute  glycerine  and  examine  the  spores  and 


PRACTICAL   WORK  253 

elaters  under  the  high  power.     Note  that  each  spore  is  divided 
into  several  cells  packed  with  chloroplasts. 

(4 )  Examine  a  prepared  longitudinal  section  through  the  centre 
of  a  spore  capsule  showing  stalk,  capsule  wall,  spores  and  elaters. 

(5)  Make  a  sketch  of    a  single  plant  of    Funaria   (or  other 
moss)  with  the  naked  eye  or  under  a  hand  lens.     In  Funaria 
note  the  short  axis  (stem)  with  a  tuft  of  brown  rhizoids  at  its 
base,  and  bearing  broad  delicate  green  leaves.     The  leafy  axis 
may  bear  a  slender  stalk  terminating  in  a  nodding  pear-shaped 
spore  capsule. 

(6)  Detach  a  single  leaf,  mount  in  a  drop  of  water  and  examine 
under  the  high  power.     Observe  that  the  leaf  consists  of  a  single 
layer  of  cells  more  or  less  alike,  except  the  midrib,  which  is  com- 
posed of  elongated  cells.     Note  the  leaf  cells  have  thin  walls  : 
each  is  lined  with  a  layer  of  cytoplasm  containing  chloroplast 
and  enclosing  a  large  central  vacuole. 

(7)  Examine  some  of  the  brown  rhizoids  in  dilute  glycerine, 
and  note  that  they  are  branching  threads  of  different  thicknesses 
composed  of  long  cylindrical  cells  with  oblique  end  walls. 

(8)  Examine  a  prepared  transverse  section  of  a  moss  stem 
showing  the  outer  cortex  of  thick-walled  cells,  the  inner  cortex 
of  thin-walled  cells,  and  the  strand  of  narrow  thin-walled  water- 
conducting  cells  hi  the  centre. 


CHAPTER  XV 

THE   SEED   PLANTS:     FORMS   AND   LIFE 
HISTORIES 

TAKEN  as  a  whole  the  Seed  Plants  represent  the  most 
complete  adaptation  to  terrestrial  life  that  we  find 
within  the  plant  kingdom ;  and,  correspondingly, 
they  are  the  plants  which  now  dominate  the  face  of 
the  earth.  On  the  vegetative  side  this  adaptation  is 
represented  by  the  broad  features  of  organ  and  tissue 
development  described  on  pp.  241-2,  which  are  common 
to  all  the  vascular  plants,  but  are  carried  to  a  higher 
pitch  of  specialisation  in  the  Seed  Plants  than  in 
the  Pteridophyta.  On  the  reproductive  side  it  is 
represented  primarily  by  the  seed  itself  and  by  the 
structures  and  processes  leading  up  to  and  accompanying 
its  development.  The  seed  is  a  direct  metamorphosis 
of  the  ovule  :  in  other  words  the  ovule  changes  directly 
into  the  seed.  And  the  ovule  is  simply  a  megaspor- 
angium  containing  a  single  megaspore,  which  has 
germinated,  not  by  producing  any  structure  external 
to  itself,  but  by  producing  the  female  gamete  or  egg 
inside  the  megaspore  as  a  result  of  the  division  of 
the  megaspore  nucleus.  The  male  gamete,  produced,  as 
in  the  heterosporous  Pteridophytes,  inside  the  micro- 
spore,  is  brought  to  the  neighbourhood  of  the  mega- 
sporangium  still  contained  in  the  microspore,  and  then, 
by  the  growth  of  a  germ  tube  put  out  from  the  microspore, 
to  the  megaspore  and  to  the  egg  itself.  The  process 


WOODY  PLANTS  AND  HERBACEOUS  PLANTS    255 

of  conjugation  of  the  gametes  is  thus  rendered  quite 
independent  of  external  water,  and  the  gametes  are 
withdrawn  from  the  danger  of  desiccation.  The 
fertilised  egg  also  germinates  inside  the  megaspore, 
where  it  produces  the  embryo,  which  develops  up  to 
a  certain  point  and  then  enters  upon  a  resting  stage. 
The  ovule  (megasporangium)  containing  the  resting 
embryo,  together  with  a  store  of  food  providing  for 
the  development  of  the  embryo  into  the  free-living 
plant,  is  called  the  seed.  Here  then  not  only  the  sexual 
generation,  but  also  the  early  stages  of  the  new  individual 
developed  from  the  fertilised  egg  are  produced  within 
the  body  and  nourished  at  the  expense  of  the  parent 
sporophyte. 

The  hundreds  of  thousands  of  existing  species  of 
Seed  Plants  are  exceedingly  various  in  form  and  life 
history.  We  can  only  consider  a  few  of  the  general 
types  into  which  these  can  be  grouped. 

First,  a  broad  distinction,  though  by  no  means  an 
absolute  distinction,  can  be  drawn  between  woody 
plants  and  herbaceous  plants.  The  subaerial  shoots 
of  woody  plants  (trees  and  shrubs)  persist  from 
year  to  year,  and  constantly  form  new  portions  of 
the  shoot  system  which  are  continuations  of  the  portions 
already  formed.  At  the  same  time  the  parts  already 
formed  in  most  cases  grow  continuously  in  thickness, 
and  the  new  layers  so  added  are  mainly  composed 
of  hard  woody  tissue.  Thus  the  whole  plant  body 
constantly  increases  in  size. 

In  the  herbaceous  plant,  on  the  other  hand,  the 
subaerial  shoot  consists  mainly  of  soft  tissue  and  is 
short-lived,  generally  dying  down  after  one  growing 
season.  The  whole  vegetative  plant  may  die  after 
one  season's  growth,  when  its  seeds  have  ripened 


256      THE   SEED   PLANTS  :    FORMS   AND   LIFE   HISTORIES 

(annual  plants],  or  the  aerial  shoots  alone  may  die  off, 
leaving  persistent  underground  shoots  (perennial  plants). 
The  underground  shoot  may  be  woody  and  persist 
indefinitely,  constantly  increasing  in  thickness  as 
well  as  in  length  like  a  woody  subaerial  shoot,  or  while 
growing  at  the  apex  from  year  to  year  it  may  con- 
tinuously die  off  behind.  In  this  chapter  we  shall 
consider  some  of  the  leading  herbaceous  types  of  seed 
plant  body. 

Erect  Herbaceous  Annual. — This  is  the  simplest 
type  of  herbaceous  plant,  but  is  not  to  be  considered 
the  most  primitive,  the  form  from  which  the  other 
types  are  derived.  On  the  contrary,  it  is  believed  by 
botanists,  on  very  good  grounds,  that  woody  perennial 
plants  are  the  most  primitive  forms  of  the  higher  plants, 
and  that  the  herbaceous,  and  finally  the  annual  plants, 
have  been  derived  from  these. 

The  body  of  an  annual  (Fig.  40,  A)  consists  of  a 
descending  portion,  the  primary  root  or  taproot,  and  its 
branches.  These  penetrate  the  soil,  fix  the  plant,  and 
absorb  water  and  inorganic  salts  from  the  soil ;  and  the 
green  shoot,  which  ascends  into  the  air  and  consists  of 
the  axes  (stems)  and  typically  flat  plate-like  organs  borne 
on  them — the  foliage  leaves. 

The  first  leaves  borne  by  the  stem — those  nearest 
the  root — are  typically  a  pair  already  formed  on  the 
embryo  in  the  seed,  and  generally  simple  in  form  : 
these  are  called  the  cotyledons.  The  part  of  the  stem 
between  the  cotyledons  and  the  top  of  the  root  is 
called  the  hypocotyl.  In  the  young  seedling  plant 

FIG.  40. — A,  diagram  of  annual  plant :  t.r.,  taproot;  hyp.,  hypocotyl ; 
cot.,  cotyledouns ;  /,  foliage  leaf ;  ax.,  axillary  bud ;  t.b.,  terminal  bud  ; 
fl.b.,  flower  bud.  B,  part  of  underground  stem  (rhizome)  of  Coral 
root  (Dentaria)  showing  fleshy  scale  leaves  (sc.l.)  of  adventitious 
roots  (r),  lateral  bud  (l.b.),  aerial  shoot  (a,s.),  arisen  from  the  ter- 


HERBACEOUS   TYPES 


257 


minal  bud.  The  rhizome  is  branching  from  two  lateral  buds.  C, 
base  of  a  young  plant  of  the  Yellow  Loosestrife  (Lysimachia  vulgaris), 
with  lateral  shoots  forming  rhizomes,  cot ,  cotyledon  ;  t.r.,  tap- 
root ;  a.s,,  aerial  shoot  from  epicotyl;  a.s2,  from  axil  of  coty- 
ledon ;  rh.,  rhizomes. 

17 


258      THE   SEED   PLANTS  :    FORMS  AND  LIFE   HISTORIES 

the  terminal  bud  is  produced  between  the  two  cotyledons. 
This  consists  of  an  apical  (primary)  shoot  meristem 
bearing  the  rudiments  of  leaves  on  its  sides.  It  grows 
out  into  the  main  shoot  of  the  seedling  (epicotyl),  the 
stem  elongating  and  the  leaves  developing  and  unfolding. 
The  levels  of  the  stem  from  which  a  leaf  (or  a  pair  or 
whorl  of  leaves)  actually  arise  are  called  nodes,  and 
the  bare  stretches  of  stem  between  the  nodes  are  inter- 
nodes.  Branches  of  the  shoot  are  developed  from 
lateral  buds,  which  are  nearly  always  developed  in  the 
angle  between  a  leaf  and  the  internode  above  the  leaf 
(axil  of  the  leaf),  and  are  hence  called  axillary  buds. 
Each  axillary  bud  repeats  the  structure  and  develop- 
ment of  the  terminal  bud.  A  bud  is  maintained  at  the 
tip  of  the  main  axis  and  at  the  tip  of  each  branch 
(terminal  buds  of  the  branches),  the  meristem  being 
constantly  renewed  by  cell  division  as  its  products 
develop  into  mature  tissue. 

Eventually  flowers  are  produced.  These  are  parts 
of  the  shoot  or  separate  shoots  (i.e.  the  whole  product 
of  a  lateral  bud)  bearing  specialised  leaves  (floral 
leaves],  some  of  which  produce  the  gametes.  Sometimes 
one  flower  is  produced  from  the  terminal  bud,  generally 
a  larger  number  from  various  axillary  buds.  A  bud 
which  is  forming  floral  leaves  is  called  a  flower  bud. 
After  the  powers  have  opened,  conjugation  of  the 
gametes  produced  by  the  floral  leaves  has  taken  place, 
and  the  zygotes  have  grown  into  embryos  within  the 
seeds,  which  are  contained  in  the  modified  remains  of 
the  flower  called  the  fruit,  the  whole  of  the  vegetative 
part  of  an  annual  plant — the  entire  root  and  shoot — 
dies,  the  life  of  the  species  being  continued  solely  in 
the  seeds,  which  under  favourable  conditions  in  due 
course  germinate  to  form  new  plants. 


RHIZOMES  259 

Rhizome-forming  Plants. — Many  herbaceous  plants, 
indeed  the  great  majority  of  species,  do  not  merely 
form  a  single  aerial  shoot  system  which  dies  at  the 
end  of  the  growing  season,  but,  in  addition  to  aerial 
shoots,  an  underground  or  surface  shoot  or  shoot  system 
which  survives  from  one  season  to  the  next,  and  is 
called  a  rhizome.  The  rhizome  may  be  formed  by  the 
whole  epicotyl  of  the  seedling  plunging  into  the  earth, 
and  growing  horizontally  instead  of  vertically  upwards 
to  form  an  aerial  shoot ;  but  in  most  cases  the  rhizome 
is  produced  by  an  axillary  bud  near  the  base  of  the 
aerial  shoot  growing  out  horizontally.  Usually  the 
rhizome  bears  scale  leaves  (Figs.  40,  B,  41,  A,  C,  sc.l.) 
which  remain  small  and  do  not  turn  green,  but  some- 
times it  bears  foliage  leaves  which  grow  up  into  the 
air  on  long  stalks.  From  the  rhizome  one  or  more 
aerial  shoots,  usually  bearing  both  foliage  leaves  and 
flower  buds,  grow  up  into  the  air.  Very  often  the 
terminal  bud  of  the  rhizome  turns  up  and  produces 
a  single  aerial  shoot  (Figs.  40,  B,  41,  B,  a.s.),  the  growth 
of  the  rhizome  itself  being  continued  by  the  growth 
of  one  or  more  axillary  buds.  Aerial  shoots  also  often 
arise  directly  from  axillary  buds. 

At  the  end  of  the  growing  season  the  aerial  shoots 
with  their  foliage  leaves  die  down,  but  the  rhizome 
remains  alive  in  a  dormant  condition  till  the  beginning 
of  the  next  growing  season,  when  growth  is  renewed 
from  terminal  and  lateral  buds  and  a  fresh  portion 
of  the  rhizome  and  new  aerial  shoots  are  produced.  The 
older  parts  of  rhizomes  commonly  die  off  and  decay.  In 
countries  like  England,  with  comparatively  mild  winters, 
the  growth  of  the  rhizome  of  many  species  continues 
throughout  the  winter  except  during"  periods  of  severe 
frost,  and  new  leaves  and  buds  are^slowly  produced, 


260      THE  SEED  PLANTS  :    FORMS  AND  LIFE  HISTORIES 


FIG.  41. — A,  young  plant  of  the  Field  Bindweed  (Convolvulus  arvensis), 
showing  the  thin  white  underground  shoots  (rh.)  which  ramify  deeply 
in  the  soil  and  make  the  plant  a  troublesome  weed.  B,  hori- 
zontally growing  rhizome  of  a  grass.  C,  plant  of  the  Chickweed 
Wintergreen  (Trientalis)  which  has  arisen  from  the  tuber  (<').  The 
new  tuber  (t1)  has  arisen  at  the  end  of  the  rhizome,  rh.,  rhizomes  ; 
r,  adventitious  roots ;  sc.l.,  scale  leaves ;  a.s.,  aerial  shoots.  (After 
Warming.) 


RHIZOMES  26l 

in  preparation  for  the  more  active  growth  and  the 
formation  of  the  aerial  shoots  in  the  next  spring. 

Roots  are  always  produced  on  rhizomes,  either  gene- 
rally distributed  on  the  surface  or  localized  at  the  bases 
of  the  aerial  shoots  (Fig.  41,  B),  or  at  the  bases  of  foliage 
leaves  arising  directly  from  the  rhizome.  Such  roots, 
arising  directly  from  a  stem,  are  sometimes  called 
adventitious  roots  as  opposed  to  the  taproot  and  its 
branches,  which  in  rhizome-growing  plants  are  short- 
lived. Organic  food  materials  (starch  or  oil — sometimes 
sugar — and  proteins)  are  stored  in  rhizomes  during  the 
winter,  and  are  "  mobilised,"  i.e.  converted  into  soluble 
forms  when  new  growth  takes  place. 

Rhizomes  may  be  short  and  compact,  the  axis  of  the 
rhizome  shoot  being  then  usually  vertical  or  oblique 
and  growth  being  very  slow  ("  male  fern,"  dandelion, 
plantain)  ;.  or  they  may  be  elongated  and  horizontal, 
often  running  for  considerable  distances  below  the 
surface  of  the  soil  (Figs.  40,  B,  41).  It  is  this  type  of 
rhizome,  with  brown  or  dingy  coloured  surface,  from 
which  the  name  (meaning  "  root-like  organ  ")  is  taken. 
But  rhizomes  are  quite  sharply  distinguishable  from 
roots  because  they  bear  leaf  structures  and  have  the 
anatomical  construction  of  stems,  not  of  roots.  The 
spread  of  many  plants,  such  as  sedges,  rushes,  some 
grasses  (e.g.  couchgrass  or  "  twitch  "),  is  due  to  this  type 
of  rhizome,  which  grows  and  branches  freely,  sending 
up  aerial  shoots  from  the  upward  turning  terminal, 
or  from  lateral  buds.  Stolons  or  runners  are  very 
thin,  horizontal,  quickly  growing  shoots,  which  run 
usually  upon  the  surface  of  the  soil  (e.g.  strawberry 
runners),  and  do  not  bear  roots.  The  terminal  bud 
eventually  turns  up,  thickens,  produces  a  rosette 
of  foliage  leaves,  strikes  roots  down  into  the  soil, 


262     THE  SEED  PLANTS  I    FORMS  AND  LIFE  HISTORIES 

and  becomes  an  independent  plant.  These  offsets 
are  regularly  used  by  gardeners  to  propagate  the 
plant. 

Some  horizontal  rhizomes,  on  the  other  hand,  are 
thick,  fleshy  and  comparatively  slow  growing  (Solomon's 
Seal),  and  these  store  considerable  quantities  of  organic 
food  reserves  during  the  winter. 

Tubers. — When  the  food  is  localised  in  a  definite 
portion  of  an  underground  stem,  the  rest  being  thin, 
the  swollen  portion  is  called  a  tuber  (Fig.  41,  C,  t). 
The  beginnings  of  tuber  formation  are  well  seen  in  the 
Chinese  Artichoke  (Stachys  tuberifera).  Many  species  of 
Stachys  have  uniform  rhizomes,  but  in  the  Chinese 
Artichoke  several  successive  internodes  are  swollen, 
the  nodes  being  somewhat  constricted  by  comparison 
with  the  internodes  and  bearing  alternating  pairs  of 
triangular  scales.  A  thinner  portion  of  the  rhizome 
follows,  the  terminal  bud  turning  up  to  produce  the 
aerial  shoot.  The  lateral  buds  which  continue  the 
growth  and  branching  of  the  rhizome  grow  out  to  form 
several  thin  internodes,  followed  by  the  thick  tuberous 
portion  in  each  case,  so  that  the  tubers  are  connected 
together  in  branching  chains. 

In  the  Potato  plant  (Solatium  tuberosum)  the  main 
shoot  is  erect,  but  thin  lateral  shoots  are  formed 
from  buds  in  the  axils  of  basal  leaves,  and  each  of  these 
grows  horizontally  or  obliquely  downwards  and  swells 
at  its  apex  into  a  potato  tuber.  When  very  young 
(about  the  size  of  a  pea)  the  tuber  is  seen  to  bear  minute 
scale  leaves,  but  these  do  not  grow  as  the  body  of  the 
tuber  swells,  and  when  the  latter  is  full  grown  their 
scars  appear,  with  the  bud  in  the  axil  of  each,  as  the 
"  eyes  "  of  the  potato.  The  "  rose  "  end  of  the  potato 
is  the  apex  of  the  tuber  where  the  "  eyes  "  are  crowded. 


CORMS  263 

When  fully  grown  the  stalk  separates  from  the  tuber. 
If  left  in  the  ground  over  the  winter,  the  eyes  grow  out 
next  year  into  the  aerial  "  haulms  "  of  new  potato 
plants.  Roots  ("  adventitious  roots  ")  are  produced 
from  the  base  of  the  haulm  just  above  the  point  at 
which  it  springs  from  the  tuber. 

Conns. — This  name  is  given  to  a  local  thickening  at 
the  base  of  the  erect  aerial  shoot,  which  carries  the  life 
of  the  plant  from  one  growing  season  to  the  next.  At 
the  end  of  the  growing  season  the  aerial  shoot  dies 
and  falls  off,  leaving  a  scar  on  the  top  of  the  corm, 
which  remains  dormant  during  the  autumn  and  winter, 
and  produces  new  aerial  shoots  from  axillary  buds  in 
the  next  growing  season.  The  garden  Crocus  is  an 
excellent  example. 

In  the  autumn,  when  the  corm  is  "  ripe  "  and  ready 
for  planting  out,  it  is  a  rounded  structure  flattened 
at  top  and  bottom  and  covered  with  thin  brown 
membranous  scales,  which  are  inserted  along  horizontal 
lines  encircling  the  corm  and  appearing  as  brown 
circular  scars  when  the  scales  are  stripped  off.  In  a 
large  vigorous  corm  there  are  two  or  three  stout  buds 
covered  with  white  scales  and  projecting  through  the 
brown  covering  at  the  top  of  the  corm.  These  can 
be  seen  to  spring  from  the  axils  of  the  brown  scales 
around  the  apical  scar  left  by  the  detachment  of  the 
aerial  shoot.  Sometimes  there  are  smaller  buds  on  the 
sides  of  the  corm  in  the  axils  of  lower  scales. 

When  the  corm  is  planted,  a  circle  of  roots,  the  be- 
ginnings  of  which  are  already  laid  down  in  the  ripe 
corm,  first  grow  out,  and  growth  soon  begins  in  the 
axillary  buds,  which  produce  the  green  shoots  that 
push  up  through  the  soil  in  February  and  flower  in 
March.  The  rudiments  of  the  foliage  leaves  and  flowers 


264      THE   SEED   PLANTS  I    FORMS  AND  LIFE  HISTORIES 

can  be  clearly  seen  in  longitudinal  section  of  a  bud 
taken  in  the  autumn.  The  food  which  supplies  the 
material  for  growth  is  stored,  largely  in  the  form  of 
starch,  in  the  tissues  of  the  corm.  When  the  aerial 
shoots  are  fully  developed  the  base  of  each  begins  to 
swell  by  the  growth  in  thickness  of  the  stem  tissue, 
and  starch,  made  from  the  sugar  produced  by  the 
foliage  leaves,  is  deposited  in  this  tissue.  Thus  a  new 
corm  is  formed  at  the  base  of  each  aerial  shoot,  and 
each  new  corm  goes  on  growing  so  long  as  the  foliage 
leaves  are  alive  (May  or  June).  During  the  summer 
or  autumn  the  new  corms,  covered  by  the  scale  leaves 
which  were  developed  at  the  base  of  the  aerial  stem, 
and  which  gradually  became  dry  and  brown,  become 
detached  from  the  old  corm,  which  is  now  finished  and 
shrivelled,  while  the  upper  part  of  the  aerial  shoot, 
also  dead,  is  detached  from  the  top  of  the  corm.  Mean- 
while axillary  buds  develop  in  the  axils  of  the  scale 
leaves  of  the  new  corms,  swell  and  produce  the  rudi- 
ments of  next  year's  foliage  leaves  and  flowers,  so  that 
by  October  the  new  corms  are  "  ripe." 

If  the  Crocuses  are  left  in  the  ground,  the  new  corms 
develop  where  they  are,  and  after  a  few  years  become 
very  crowded,  so  that  the  new  corms  are  apt  to  be 
small  and  feeble,  often  possessing  only  one  new  axillary 
bud  which  is  strong  enough  to  flower.  That  is  why 
a  better  crop  of  flowers  is  produced  if  corms  are  taken 
up  as  soon  as  the  leaves  die,  and  only  the  most  vigorous 
selected  for  replanting. 

Other  examples  of  plants  which  go  through  the 
winter  in  the  form  of  corms  are  the  Meadow  Saffron 
(Golchicum  autumnale) — found  in  meadows  and  woods 
especially  in  the  west  of  England — which  produces  its 
new  corm  in  the  same  way,  but  at  the  side  of  the  old 


BULBS.      PERENNATION  265 

one  ;  the  bulbous  buttercup  (Ranunculus  bulbosus)  ; 
common  in  dry  pastures  ;  and  Cyclamen. 

Bulbs. — The  life  history  of  a  plant  which  perennates 
by  means  of  bulbs  is  very  similar  in  general  features 
to  that  of  the  corm-forming  plants,  the  distinction  being 
that  in  the  bulb  the  store  of  food  is  contained  in  the 
scale  leaves  (bulb  scales)  of  the  winter  shoot,  the  stem 
being  represented  by  a  disc  at  the  base  of  the  bulb. 
On  this  are  inserted  the  outer  membranous  brown 
protective  scales,  and  the  massive  bulb  scales  which 
form  the  greater  part  of  the  body  of  the  bulb. 

In  the  Tulip,  which  is  a  suitable  type  for  study, 
there  is  however  another  difference.  The  aerial  flowering 
shoot  is  produced  from  the  terminal  bud  of  the  bulb, 
not,  as  in  Crocus,  from  axillary  buds.  The  new  bulbs 
are  formed,  not  by  the  swelling  of  part  of  the  aerial 
shoot,  but  by  the  formation  of  buds  in  the  axils  of  the 
bulb  scales.  These  buds  gradually  swell  during  the 
growing  season,  as  the  old  bulb  scales  become  emptied, 
and  thus  produce  the  new  bulbs  inside  the  old  one. 
It  is  even  more  important  to  dig  up  and  remove  tulip 
bulbs  from  the  soil  at  the  end  of  the  growing  season 
if  the  crop  is  to  be  maintained. 

Some  bulb-forming  plants  (onions,  lilies,  etc.)  also 
produce  under  certain  conditions  small  bulbs  called 
bulbils  in  place  of  all  or  some  of  the  flowers.  These 
become  detached,  fall  to  the  ground,  and  in  suitable 
soil  strike  root  and  reproduce  the  plant. 

Perennation,  Multiplication  and  Spreading.  —  The 
types  of  more  or  less  specialised  underground  shoots 
described — rhizomes,  tubers,  corms  and  bulbs — all  serve 
to  carry  the  plant  on  in  the  vegetative  form  from  one 
growing  season  to  the  next,  for  they  can  exist  unharmed 
in  a  dormant  condition  in  the  soil  through  the  winter, 


266      THE   SEED   PLANTS  :    FORMS   AND   LIFE  HISTORIES 

when  the  aerial  shoots  have  died  ;  and  they  store  food 
from  which  the  next  season's  growth  is  started. 
Primarily  these  underground  shoots  are  thus  a  means 
of  perennation.  Many  plants  which  flower  in  early 
spring  have  bulbs,  corms  or  rhizomes,  and  are  able  to 
make  new  growth  quickly  because  of  the  great  store 
of  organic  food  at  their  immediate  disposal. 

When  a  rhizome  branches  and  the  old  parts  die  off 
so  that  the  branches  are  separated,  the  plant  is  pro- 
pagated vegetatively,  i.e.  the  number  of  individuals  is 
multiplied  by  purely  vegetative  means,  just  as  we 
saw  happened  in  Mosses  and  Liverworts,  and  as  happens 
also  in  Spirogyra  when  the  cell  thread,  after  increasing 
in  length  by  cell  division  and  growth,  splits  into  lengths, 
and  produces  a  corresponding  number  of  new  plants. 

It  is  essentially  the  same  process  when  a  plant 
produces  many  tubers,  or  a  Crocus  corm  several  aerial 
shoots,  at  the  base  of  each  of  which  a  new  corm  is 
formed,  or  when  a  bulb  produces  several  new  lateral 
bulbs.  Though  the  plant  is  thus  multiplied,  the  new 
individuals  are  developed  close  together,  and  compete 
for  the  same  space,  light  and  food — the  species  is  not 
dispersed.  But  dispersal  occurs,  though  gradually, 
if  the  underground  stems  spread  widely,  producing 
new  roots  and  new  aerial  shoots  from  their  tips.  The 
advantage  of  the  bulb  or  corm  is  that  it  is  better  pro- 
tected from  desiccation  than  many  rhizomes,  and  is 
thus  particularly  well  adapted  to  plants  living  in 
climates  with  a  very  dry  season.  South  Africa,  for 
instance,  and  the  Mediterranean  region  have  a  large 
number  of  bulb  and  corm  producing  species.  The 
British  climate  has  no  very  dry  season,  and  there  are 
many  more  rhizomatous  plants  which  are  able  to 
stand  our  comparatively  mild  winters  perfectly  well, 


SHOOTS   AS   POTENTIAL   INDIVIDUALS  267 

though  the  rhizomes  are  not  specially  well  protected, 
and  often  grow  slowly  throughout  the  winter. 

The  methods  of  vegetative  propagation  in  the  higher 
plants  show  that  any  shoot  which  has  the  power  of 
rooting  at  its  base  may  be  regarded  as  a  potential 
individual.  In  artificial  propagation  by  means  of 
cuttings,  a  branch  shoot  is  cut  off  and  the  cut  end 
stuck  into  damp  soil.  Roots  are  produced  from  this 
surface,  and  the  detached  shoot  becomes  a  new  plant. 
This  extensive  power  of  producing  new  individuals 
vegetatively  is  a  character  which  separates  the  highest 
plants  very  sharply  from  the  highest  animals.  The 
bodies  of  the  latter  are  very  highly  integrated,  i.e. 
they  form  very  closely  knit  wholes,  so  that  none  of  their 
parts  can  live  independently,  while  the  bodies  of  plants, 
with  their  indefinite  growth  and  number  of  branches, 
constantly  repeating  the  structure  of  the  original 
shoot,  and  able  under  suitable  conditions  to  live  inde- 
pendently, do  not  form  nearly  such  highly  integrated 
individuals.  This  is  largely  connected  with  the  fact 
that  the  bulk  of  their  bodies  is  composed  of  com- 
paratively undifferentiated  living  cells  which  are  not 
so  far  removed  from  germ  cells  as  those  of  the  more 
highly  specialised  tissues  of  the  higher  animals.  (Cf. 
pp.  206-7.) 

PRACTICAL   WORK. 

Annual  Herbaceous  Plant. 

(i)  Examine  a  small  herbaceous  plant  with  a  single  main  shoot 
and  a  taproot.  Make  a  sketch  of  the  plant  showing  the  following 
parts  :  taproot,  branch  roots,  hypocotyl,  cotyledons,  nodes,  intern  odes, 
leaves,  leaf  veins,  terminal  buds,  axillary  buds,  flowers  or  flower  buds. 

[Any  small  quickly  developing  annual  is  suitable  if  it  retains 
its  cotyledons  till  it  produces  flowers.  Chickweed  (Stellaria 
media]  or  one  of  the  annual  Veronicas — V.  agrestis,  Tottrnefortii 
or  hedercefolia — does  very  well.] 


268      THE   SEED   PLANTS  :    FORMS   AND   LIFE   HISTORIES 

Rhizomes. 

(2)  Examine    and    sketch    a   portion    of   the    Couchgrass    or 
"  Twitch  "   (Agropyrum  repens)   or  any  grass  of  similar  habit. 
The  main  stem  of  the  plant  is  the  rhizome,  bearing  scale  leaves 
and  roots.     The  aerial  leafy  shoots  arise  from  buds  in  the  axils 
of  the  rhizome  scales.     Branching  of  the  rhizome  is  secured  by 
the  outgrowth  of  other  axillary  buds  which  grow  horizontally. 
Note  the  abundant  production  of  roots  at  the  bases  of  the  aerial 
shoots,  and  the  sharp  points  formed  by  the  folded  scales  covering 
the  terminal  buds  of  the  rhizome  branches.     These  can  penetrate 
stiff  clay  and  often  bore  through  objects  such  as  potatoes  which 
they  may  encounter. 

(3)  Compare  the  stout,  more  slowly  growing  rhizome  of  Solo- 
mon's Seal  (Polygonatum) .     The  scales  which  cover  the  terminal 
bud  fall  off,  leaving  only  brown  circular  scale  scars  on  the  surface 
of  the  rhizome.     The  single  aerial  shoot  is  formed  each  year  by 
the   turning  up   of   the   terminal   bud.     The   continuation   and 
branching  of  the  rhizome  are  secured  by  other  buds  formed  in 
the  spring.     Compare  the  early  spring  condition  with  the  summer 
condition  (museum  or  herbarium  specimens). 

Tubers. 

(4)  Sketch    the    tuberously    thickened     branching    rhizomes 
of   the   Chinese   Artichoke   (Stocky s  tuberifera).     Note   the   thin 
portions   of  rhizome   preceding   and   following   the   tubers,    the 
constricted  nodes,   with  triangular  scales  and  axillary  buds  in 
opposite  pairs,   alternating"  on   succeeding  nodes,   and   the   up- 
wardly turned  terminal  bud. 

(5)  Examine  potato  tubers — a  seed  potato  ready  for  planting, 
and   a  sprouted   potato   are   most  instructive.     Mark   on   your 
drawings  the  scar  of  the  tuber  stalk,  the  eyes,  the  terminal  bud, 
and  in  the  sprouts  the  scale  leaves,  developing  foliage  leaves  and 
adventitious  roots.     Examine  a  specimen  showing  early  stages 
in  the  formation  of  tubers  with  the  stalk  arising  from  the  axil  of 
a  leaf  on  the  parent  plant  and  the  minute  scales  on  the  very 
young  tuber. 

Conn. 

(6)  Crocus,  autumn  stage.     Carefully  peel  off  the  dry  mem- 
branous scales  and  note  that  they  are  inserted  one  above  the 
other,  the  lower  ones  overlapping  the  upper  on  the  side  of  the 
corm,    leaving   brown   circular   scars   when   detached.     Identify 
the  stem  scars  at  the  top  and  bottom  of  the  corm.     The  former 
is  the  scar  of  last  spring's  flowering  and  leafy  shoot,  of  which 


PRACTICAL   WORK  269 

the  present  corm  is  the  swollen  base.  The  scar  at  the  bottom  is 
the  attachment  of  the  present  to  last  year's  corm,  which  has 
shrivelled  and  fallen  off.  Note  the  circle  of  young  roots  ready 
to  grow  out  round  this  scar. 

With  a  sharp  knife  cut  longitudinally  through  the  corm,  taking 
care  to  pass  through  the  terminal  scar  and  through  one  of  the  buds 
at  the  top  of  the  corm,  and  note  in  the  bud  (a)  the  external  scale 
leaves,  (b)  the  rudiments  of  the  foliage  leaves,  (c)  one  or  more  flower 
buds.  Make  a  drawing  of  the  cut  face  showing  the  vascular 
cylinders  passing  to  the  top  scar  and  to  the  buds.  Test  the  cut 
surface  for  starch. 

(7)  Compare  the  flowering  stage  of  the  Crocus  plant,  and  note 
the  swelling  at  the  bases  of  the  aerial  shoots  which  will  become 
the  new  corms. 

Bulb. 

(8)  Tulip,  autumn  stage.     Cut  longitudinally  exactly  through 
the  centre  of  the  bulb.     Note  the  disc-shaped  stem  at  the  base 
with  point  of  attachment  to  the  old  bulb,  rudiments  of  roots, 
membranous  protective  scales,  fleshy  bulb  scales,  terminal  bud 
of   aerial  shoot  with  rudiments  of  foliage  leaves  and  flower. 
Look  for  a  small  bud  or  buds  in  the  axils  of  the  bulb  scales. 
Test  the  cut  bulb  scales  with  iodine. 

(9)  Examine  the  flowering  stage  and  note  the  roots  grown  out 
from  the  edge  of  the  bulb  stem,  and  the  flowering  shoot  developed 
from  the  summit  of  the  bulb.     Cut  longitudinally  through  the 
bulb  and  note  the  flabby  depleted  bulb  scales,  and  the  new  bulb 
or  bulbs  arisen  from  the  axillary  bud  or  buds  seen  in  (8). 

(10)  Examine  any  other  examples  of  underground  shoots  or 
of  vegetative  propagation  that  may  be  available. 


CHAPTER   XVI 

THE  TISSUE    ELEMENTS    OF    SEED    PLANTS 

BEFORE  we  study  the  microscopic  structure  of  the  vege- 
tative organs  of  the  Seed  Plants  it  is  desirable  to  have 
some  knowledge  of  the  different  elements  of  which 
the  tissues  are  built  up,  i.e.  of  the  different  kinds  of 
more  or  less  specialised  cells  ot  which  the  bodies  of  the 
higher  plants  are  composed.  If  we  can  recognise  the 
elements  of  the  different  tissues  we  meet  with  in  the 
structure  of  root,  stem  or  leaf  we  shall  the  more  readily 
grasp  the  way  in  which  these  tissues  are  arranged  and 
interrelated  in  these  organs. 

All  the  various  elements  that  make  up  the  tissues  of 
a  vascular  plant  are  derived  from  embryonic  cells  such  as 
are  found  in  the  primary  meristems  at  the  tips  of  the 
branches  of  the  root  and  shoot  (see  Chapter  VI,  p.  102). 
Most  of  the  body  of  a  herbaceous  plant,  the  "  soft  " 
tissues,  are  composed  of  living  cells  with  cellulose  wall 
of  moderate  thickness,  and  a  large  central  vacuole, 
so  that  the  cytoplasm  forms  a  layer  on  the  cell  wall. 
The  development  of  such  cells  from  the  embryonic 
cells  was  described  in  detail  on  p.  107.  This  sort  of 
tissue  is  called  parenchyma,  and  it  forms  most  of  the 
"  ground  tissue  "  of  the  plant  body  ;  it  is  also  found 
associated  in  masses,  strands  or  single  cells  with  the 
more  specialised  tissues.  The  mesophyll  of  the  leaf 
(pp.  115)  is  composed  of  this  sort  of  cell  in  which  the 
numerous  plastids  embedded  in  the  cytoplasm  have 


LIVING   PARENCHYMA 


271 


become  chloroplasts.  Certain  adult  living  cells,  how- 
ever, differ  in  one  way  or  another  from  this  typical 
structure. 


A 


m, 


FIG.  42. — Development  of  adult  living  cells  (parenchyma)  from 
embryonic  cells.  A,  embryonic  (meristeraatic)  cells.  B,  begin- 
ning of  vacuolation.  C,  fully  grown  cells  (here  much  elongated) 
with  characteristic  large  central  vacuole ;  c.w.,  cell  wall ;  cyt., 
cytoplasm ;  n,  nucleus ;  no.,  nucleolus ;  v,  vacuole. 


272  TISSUE   ELEMENTS   OF   SEED   PLANTS 

Cells  rich  in  Protoplasm.  Secretory  Cells  :  Protein 
Cells. — Some  cells  remain  densely  filled  with  protoplasm 
even  when  adult,  or  at  least  contain  much  more 
cytoplasm  in  proportion  to  cell  sap  than  the  average 
tissue  cell.  The  nucleus  is  large  and  conspicuous. 
Prominent  among  these  are  the  cells  (gland  cells  or 
secretory  cells)  whose  special  function  is  the  secretion 
of  a  definite  substance,  such  for  instance  as  the  cells 
of  the  nectaries  of  flowers  which  secrete  a  sugary  fluid — 
nectar ;  as  also  the  cells  which  produce  a  particular 
enzyme,  such  as  are  found  in  seeds  where  large  quantities 
of  stored  food  material  are  brought  into  a  soluble 
form  in  a  short  time,1  and  diastase,  cytase,  or  protease 
is  produced  in  comparatively  large  quantities.  The 
glands  of  insectivorous  plants,  also,  secrete  proteolytic 
enzymes  which  bring  the  proteins  of  their  victims' 
bodies  into  soluble  forms.  Secretory  cells,  when  active, 
have  large  conspicuous  nuclei,  and  are  often  destitute 
of  a  vacuole.  They  closely  resemble  in  structure  and 
appearance  the  cells  of  the  glandular  epithelium  of 
animals  (Fig.  5,  B)  whose  function  is  the  same. 

A  kind  of  cell  which  resembles  these  in  appearance 
and  structure,  and  may  indeed  be  said  to  belong  to 

1  It  is  to  be  understood  that  the  production  of  such  substances 
is  by  no  means  the  monopoly  of  "  secretory  cells."  They  may  be 
produced  and  are  produced  in  the  most  various  living  cells.  The  cells 
called  secretory  are  those  which  are  specialised  in  this  direction,  and 
continuously  produce  large  quantities  of  the  substances  in  question. 

FIG.  43. — Living  tissue  elements  of  the  Seed  Plant.  A,  secreting 
(glandular)  hair  arisen  from  epidermis  of  leaf.  The  four  cells 
of  the  hair  are  richly  protoplasmic  and  have  conspicuous  nuclei. 
Compare  with  living  cells  of  epidermis  and  mesophyll  below 
which  contain  much  less  protoplasm ;  s.h.,  secretory  hair ;  «, 
nucleus;  v,  vacuole;  ep.,  epidermis ;  me s,  mesophyll;  cA.,chloroplast. 
B,  secreting  cells  from  seed  of  Rye  (Secale),  with  dense  granular 
cytoplasm  and  conspicuous  nucleus  (n).  C,  part  of  sieve  tube 
in  longitudinal  section  showing  two  sieve  plates  (covered  with 
callose)  and  funnel-shaped  slimy  cell  contents  (contracted  away 
from  wall).  On  the  right  two  companion  cells  (protein  cells) 
with  dense  cytoplasm  and  conspicuous  nuclei.  On  the  left  an 


ACTIVE    LIVING   CELLS 


273 


Tries. 


ordinary  parenchyma  cell.  D,  sieve  plate  and  companion  cell 
as  seen  in  cross-section.  E,  epidermal  cell  with  large  vacuole. 
but. large  conspicuous  nucleus  in  contact  with  local  thickening 
of  outer  cell  wall  which  is  being  formed  ;  p,  pit  in  lateral  wall. 
F,  large  water  bladder  representing  a  single  much  swollen  cell 
of  the  epidermis,  with  little  cytoplasm  but  large  nucleus ;  ep., 
ordinary  epidermal  cells  ;  mes.,  mesophyll. 

18 


274  TISSUE   ELEMENTS   OF   SEED   PLANTS 

the  secretory  cell  type,  is  the  protein  cell  (Fig.  43,  C), 
which  is  constantly  associated  with  the  cells  (sieve  tubes) 
that  conduct  organic  food  substances.  The  exact 
functions  of  these  protein  cells  are  not  understood  : 
perhaps  they  secrete  enzymes  which  act  on  the  substances 
passing  along  the  sieve  tubes.  Protein  cells  have 
large  conspicuous  nuclei,  and  are  very  rich  in  protein 
contents. 

Sieve  Tubes  are  the  main  channels  of  transport  of 
organic  substances  from  one  part  of  the  plant  to  another. 
Each  sieve  tube  consists  of  a  row  or  chain  of  elongated 
cells  (Fig.  43,  C),  comparable  with  the  medullary  cell 
chains  of  Fucus.  The  cross  walls  separating  the 
successive  cells  of  the  chain  early  become  perforated  by 
holes — eaten  out  as  it  were  by  the  action  of  an  enzyme 
— 'Which  in  extreme  cases  may  be  as  much  as  5  //,  in 
diameter,  and  through  which  pass  relatively  coarse 
"slime  strands"  connecting  each  cell  with  the  next 
cell  of  the  tube.  The  perforated  cross  wall  is  called 
a  sieve  plate  (Fig.  43,  D).  The  vacuoles  of  the  sieve 
tube  cells  become  filled  with  a  highly  nitrogenous 
thin  mucilage.  The  layer  of  cytoplasm  lining  the  walls 
remains,  but  the  nucleus  disappears,  and  the  cells  can 
hardly  be  considered  "  living  "  in  the  full  sense.  On 
the  sieve  plate  there  is  eventually  formed  a  mass  of 
a  carbohydrate  substance  called  callose  which  blocks 
the  holes  in  the  plate  (Fig.  43,  C).  Callose  refracts 
light  strongly  and  takes  a  deep  colour  with  certain 
stains.  The  blocking  of  the  holes  by  the  callose 
apparently  interrupts  the  transport  of  substances 
along  the  tube ;  later  the  callose  is  dissolved,  and 
ultimately  the  sieve  tube  becomes  empty  and  no  longer 
functional,  for  instance  in  the  parts  of  woody  plants 
which  are  several  years  old. 


WATER   TISSUE  275 

Each  sieve  tube  cell  has  one  or  more  companion  cells 
alongside  of  it  (Fig.  43,  C).  These  are  typical  "  protein 
cells  "  (see  above).  In  most  Seed  Plants  they  are  cut 
off  by  division  from  the  mother  cell  of  each  sieve  tube 
segment  at  an  early  stage  of  its  development,  before 
the  sieve  tube  cell  acquires  its  special  character. 

The  sieve  tubes  are  certainly  the  main  conducting 
channels  of  organic  nitrogenous  substances  (amino-acid 
compounds,  amides  and  perhaps  the  simpler  proteins) 
and  of  sugars,  but  we  know  very  little  indeed  of  the 
manner  in  which  the  transport  takes  place. 

Living  Cells  poor  in  Protoplasm.  Water  Tissue. — At 
the  other  extreme  from  secretory  and  protein  cells  are 
water  cells,  which  have  a  layer  of  cytoplasm  lining  the 
wall  very  thin  in  comparison  with  the  size  of  the  cell 
(Fig.  43,  E,  F),  the  vacuole  being  filled  with  a  very 
watery  sap,  i.e.  an  extremely  dilute  solution.  These 
cells  are,  however,  only  an  extreme  form  of  the  typical 
thin-walled  parenchymatous  cell.  The  bodies  of 
succulent  plants  consist  mainly  of  such  thin-walled 
water  cells,  but  in  the  typical  succulents  (Cacti]  the 
cells  contain  a  large  quantity  of  pentosans  (substances 
having  a  similar  relation  to  pentose  sugars  that  the 
celluloses  have  to  the  hexose  sugars)  which  attract 
water  strongly.  The  active  cells,  for  instance  the 
photosynthetic  cells  near  the  surface  of  the  succulent 
stem  or  leaf,  can  draw  upon  the  store  of  water  in  the 
water  tissue. 

The  epidermal  cells  act  as  water  cells  in  an  ordinary 
leaf  and  provide  a  small  supply  of  water  which  is  drawn 
upon  by  the  mesophyll  cells  if  they  are  losing  water 
to  the  air  more  quickly  than  they  can  be  supplied 
from  the  water-conducting  system.  In  some  cases 
isolated  epidermal  cells  are  many  times  the  size  of 


2y6  TISSUE   ELEMENTS   OF   SEED   PLANTS 

their  neighbours,  and  act  as  special  water  bladders 
(Fig.  43,  F).  And  the  leaves  of  certain  plants  growing 
in  places  where  they  are  specially  liable  on  occasion  to 
lose  water  quicker  than  it  can  be  supplied  have  a 
many  layered  thin-walled  epidermis,  and  this  massive 
water  tissue  loses  water  to  the  mesophyll  and  partially 
collapses,  bellows  fashion,  under  such  conditions.  Thick 
leaves  often  possess  a  central  water  tissue. 

Thick-Walled  Cells— Pits. — Many  of  the  living  cells  of 
a  plant  thicken  their  walls  considerably  after  the  cell 
has  grown  to  its  definitive  size.  This  thickening  of  the 
wall  is  often  general  or  uniform  over  the  whole  wall 
surface,  and  may  be  carried  so  far  that  the  cell  cavity  is 
greatly  reduced  in  volume  (Fig.  44,  C,  D,  E).  Successive 
layers  of  cellulose  are  laid  down  by  the  cytoplasm,  and 
the  layering  (stratification)  of  the  wall  can  often  be 
clearly  seen  under  the  microscope  (Fig.  44,  B,  C). 
This  appearance  is  supposed  to  depend  upon  the  smaller 
or  greater  amount  of  water  contained  in  successive 
layers  of  wall,  making  the  cellulose  more  or  less  solid 
in  consistency. 

The  continuity  of  the  layers  is,  however,  generally 
interrupted  at  certain  spots,  where  the  original  wall, 
formed  at  cell  division  (middle  lamella),  is  not  added  to. 
These  thin  places  in  the  wall  are  called  pits.  Pits  are 
always  formed  opposite  to  one  another  in  adjacent 
cells  of  a  thick- walled  tissue  (Fig.  43,  E,  p),  so  that 
the  cell  cavities  are  only  separated  by  the  thickness 
of  the  middle  lamella  (pit  membrane).  A  pit  is  very 
commonly  circular  in  section,  so  that  it  appears  as  a 
lighter  circular  area  when  the  wall  is  seen  in  surface  view 
under  the  microscope.  The  walls  of  a  pit  are  commonly 
perpendicular  to  the  middle  lamella,  so  that  the  pit  is 
of  equal  diameter  from  top  to  bottom  ("  simple  pits  "). 


BORDERED    PITS 


277 


But  in  some  cells,  particularly  the  vessels  and  tracheids 
of  the  water-conducting  system  (see  below),  the  pit  is 
narrower  where  it  opens  into  the  general  cell  cavity 
than  at  the  bottom  on  the  pit  membrane.  The  walls 
thus  overhang  the  pit,  which  is  then  said  to  be  bordered 


B 


Iff? 


FIG.  44. — Mechanical  tissue  elements  of  the  Seed  Plant.  A,  collen- 
chymatous  cells  in  longitudinal  section,  showing  great  thickening 
of  parts  of  the  walls  forming  longitudinal  "  pillars  "  of  wall 
substance.  B,  the  same  in  cross-section  ;  w,  thickened  walls. 
C,  thick  walled  "  stone  cell  "  with  branching  pits.  D,  part 
of  three  fibres  in  longitudinal  section.  E,  fibres  in  cross-section. 

(Fig.  45,  I).  Some  bordered  pits  are  not  included  in 
the  general  thickness  of  the  cell  wall,  but  are  built  out, 
so  to  speak,  into  the  cell  cavity  in  the  form  of  a  flattish 
dome,  with  the  pit  opening  at  its  summit.  The  two 


278  TISSUE   ELEMENTS   OF   SEED   PLANTS 

opposite  pits  of  adjacent  cells  then  form  a  lens-shaped 
structure  on  the  cell  wall.  The  extreme  example  of 
this  type  of  bordered  pit  is  seen  in  the  tracheids  (water- 
conducting  cells)  of  conifers,  for  instance  the  common  pine. 

The  fine  threads  of  cytoplasm,  originally  the  achro- 
matic spindle  threads  (see  p.  106),  connecting  the  cyto- 
plasm of  adjacent  cells,  are  sometimes,  though  not  always, 
confined  to  the  pit  membranes  of  thick-walled  cells. 
The  existence  of  pits  greatly  facilitates  the  passage 
of  substances  in  solution  from  one  thick-walled  cell  to 
another,  and  through  the  pits,  no  doubt,  the  sugar 
which  supplies  the  carbohydrate  material  of  which  the 
thick  cell  wall  is  made  mainly  enters  the  cell.  We  do 
not  know  what  determines  the  formation  of  a  pit  at 
any  given  spot  on  the  cell  wall,  though  in  some  cases 
it  is  clearly  connected  with  the  presence  of  a  bundle 
of  cytoplasmic  threads  passing  through  the  middle 
lamella  which  in  some  way  must  arrest  the  deposition 
of  cellulose  at  that  spot.  The  formation  of  thick  walls 
in  general  is  an  expression  of  excess  of  soluble  carbo- 
hydrate substance  in  the  cell,  which  is  condensed  and 
added  to  the  wall  as  cellulose.  In  many  cases,  indeed, 
thick-walled  tissue  is  of  no  particular  use  to  the  plant, 
but  is  simply  formed  as  a  result  of  carbohydrate  excess, 
when  photosynthesis  is  unchecked,  but  there  is  not  a 
sufficient  supply  of  salts  to  form  proteins  for  the 
manufacture  of  new  protoplasm.  Thus  most  plants 
living  in  dry  climates  form  great  masses  of  thick-walled 
tissue.  But  thick-walled  cells  may  be  of  great  use  to 
the  plant,  since  they  lend  rigidity  and  toughness  to 
its  body  (mechanical  tissue). 

Local  Thickening  of  Cell  Walls.— Sometimes  the 
thickening  of  a  cell  wall  is  not  uniform  over  the  whole 
surface,  but  local :  for  instance  in  a  box-shaped  cell 


MECHANICAL   OR   SUPPORTING   CELLS  279 

one  wall  only  may  be  thickened.  The  external  walls 
of  the  epidermal  cells  covering  the  shoot  are  generally 
considerably  thicker  than  the  lateral  and  inner  walls, 
and  sometimes  the  outer  wall  is  very  thick  in  the  centre 
and  lens-shaped  (Fig.  43,  E).  In  collenchyma,  the  sup- 
porting tissue  of  elongated  living  cells  commonly  met 
with  just  below  the  epidermis  in  herbaceous  stems  and 
in  the  midribs  of  leaves,  the  thickening  is  confined  to 
the  corners  of  the  cells,  so  that  longitudinally  running 
pillars  of  cellulose  are  formed,  connected  with  the 
neighbouring  pillars  by  thin  membranes  (Fig.  44,  A,  B). 
Thus  there  is  a  certain  amount  of  "  give "  under 
horizontal  strain  in  this  tissue.  In  many  cells  ribs  or 
other  local  projections  of  various  shapes  are  laid  down 
on  the  inner  surface  of  the  wall.  The  spiral  thickenings 
on  the  walls  of  the  elaters  of  Pellia,  and  various  similar 
thickenings  in  the  cells  of  Seed  Plants  (Fig.  45,  A-G), 
are  cases  in  point. 

Dead  Cells. — In  many  cells  of  the  plant  body  the 
protoplasm  dies  after  the  development  of  the  cell 
is  completed,  the  wall  alone  remaining.  While  these 
dead  cells  can  no  longer  take  an  active  part  in  the  life 
of  the  plant,  they  often  form  an  integral  part  of  its 
structure,  and  many  of  them  are  of  essential  importance. 
The  two  chief  functions  they  carry  out  are  those  of 
mechanically  strengthening  the  plant  body  and  of 
water  conduction.  The  chief  mechanical  or  supporting 
tissues  are  composed  of  thick-walled  elongated  cells 
with  long  tapering  ends,  which  fit  between  one  another, 
so  that  the  tissue1  is  very  tough  and  not  easily  broken 
by  strains.  Such  cells  are  called  fibres  (Fig.  44,  D,  E). 
The  wall  of  the  fibre  is  sometimes  very  thick,  almost 
obliterating  the  cell  cavity.  It  is  usually  penetrated 
by  narrow  pits,  which  serve  to  bring  the  sugars  used 


280  TISSUE   ELEMENTS   OF   SEED   PLANTS 

for  thickening  into  the  cell.  The  protoplasm  generally 
dies  when  the  thickening  is  complete.  The  fibres  are 
commonly  found  in  bands  or  masses  not  very  far  from 
the  surface  of  a  herbaceous  stem  and  in  a  similar  position 
in  the  midrib  and  larger  veins  of  the  leaf,  but  they 
also  form  a  large  part  of  the  wood  of  many  woody 
plants,  for  instance  the  oak,  to  which  they  give  great 
hardness  and  toughness.  This  depends  partly  on 
changes  in  the  cell  wall  (see  below).  Fibrous  tissue 
is  used  for  making  linen,  canvas  and  paper,  in  which 
the  fibres  are  woven  together  to  make  the  textile  fabric. 
The  walls  of  the  fibres  in  linen  and  the  best  paper  are 
nearly  pure  cellulose. 

Other  thick- walled  cells  are  box-shaped,  spherical 
or  oval,  and  when  the  walls  are  very  hard  they  are 
called  stone  cells,  which  form  the  substance  of  the  stone 
in  stone  fruits  (plum,  cherry,  etc.),  and  isolated  cells 
or  nests  of  cells  in  the  flesh  of  a  gritty  pear.  Stone 
cells  frequently  have  beautiful  branched  pits  (Fig.  44,  C), 
formed  by  the  meeting  and  coalescence  of  several 
different  pits  as  the  inner  surface  of  the  cell  is  diminished 
by  the  progressive  general  thickening  of  the  wall. 

Masses  of  thick-walled  cells  near  the  surface  of  an 
organ  may  also  have  an  important  function  in  checking 
loss  of  water  by  evaporation. 

The  water-conducting  cells  of  a  plant  are  called 
tracheids  and  vessels  (Fig.  45).  These  are  dead  cells 
with  lignified  walls  (see  below),  which  form  continuous 
water-conducting  channels  throughout  the  plant  parallel 
with  the  sieve  tubes  A  tracheid  is  usually  an  elongated 
cell  whose  wall  may  be  thickened  in  various  ways. 

(a)  Spiral  tracheids  have  one  or  more  lignified  bands 
of  wall  substance  (Fig.  46)  laid  down  on  the  inner 
surface  of  the  wall  and  running  round  and  round  the 


SPIRAL   AND   ANNULAR   TRACHEIDS 


28l 


cell  through  its  entire  length.  The  successive  turns  of 
the  spiral  are  at  first  in  close  lateral  contact  (Fig.  45,  A). 
The  wall  itself,  apart  from  the  thickening,  usually 
remains  thin  and  cellulose. 


H 


FIG.  45. — Water-conducting  tissue  elements.  A,  spiral  tracheid,  not 
yet  elongated,  surface  view.  B,  elongation  beginning,  separating 
coils  of  spiral  thickening.  C,  portion  of  B  more  highly  magni- 
fied, showing  nucleus  and  cytoplasm  still  alive.  D,  part  of 
tracheid  with  coils  of  spiral  further  separated.  E,  ditto  of 
annular  tracheid.  F,  spiral  tracheid  with  cavity  nearly 
obliterated  owing  to  pulling  out  of  spiral  thickening  till  it  no 
longer  supports  the  thin  wall.  G,  ditto  in  annular  tracheid. 
H,  part  of  scalariform  vessel.  I.  bordered  pits :  in  (a)  sectional 
view,  (b)  surface  view  (corresponding  points  joined  by  horizontal 
lines).  J,  part  of  pitted  vessel.  K,  part  of  reticulate  vessel. 

(b)  Annular  tracheids  (Fig.  45,  E,  G)  are  similar,  but 
have  the  thickening  in  the  form  of  separate  rings  of 
lignified  substance,  set  one  above  the  other,  at  first 
in  close  contact. 


282  TISSUE   ELEMENTS   OF   SEED   PLANTS 

Both  these  types  are  formed  in  tissue  which  is  still 
elongating,  and  the  stretching  of  the  thin  cell  wall 
separates  the  coils  of  the  spiral  or  the  annuli,  as  the 
case  may  be  (Fig.  45,  B — G). 

(c)  Pitted  tracheids  have  general  thickening  of  the 
cell  wall,  but  with  numerous  pits  often  set  so  close 
together  (Fig.  45,  J)  that  only  ribs  of  thickening  are 
formed  between  them.  The  pits  are  generally  bordered, 
often  with  greatly  overhanging  walls  (Fig.  45,  I).  When 
the  pits  are  elongated  and  horizontal  in  direction,  as 
seen  in  surface  view,  running  across  one  face  of  a  poly- 


FIG.  46. — Part  of  a  spiral  tracheid  very  highly  magnified  and  seen 
in  longitudinal  section,  showing  attachment  of  the  spiral  thickening 
band  to  the  thin  wall. 

gonal  tracheid,  so  that  there  are  horizontal  bars  of 
thickening  between  successive  pits,  like  the  rungs  of  a 
ladder,  the  tracheid  is  called  scalariform  (Fig.  45,  H). 
When  the  pits  are  angular  and  variable  in  outline,  so 
that  there  is  a  network  of  thickening  between  them, 
we  have  the  reticulate  type  (K).  The  pits  are,  however, 
very  often  circular,  oval  or  polygonal  in  outline  and 
very  closely  set  (J). 

A  vessel  has  its  walls  lignified  and  thickened  just 
like  a  tracheid,  but  it  consists   of    a    row  or   chain  of 


LIGNIFICATION  283 

cells  with  open  communication  between  them,  owing  to 
the  cross  walls  having  been  dissolved  wholly  or  partially 
by  the  protoplasm  before  the  cells  were  fully  developed. 
A  vessel  is  thus  comparable  with  a  sieve  tube  in  that 
both  are  conducting  structures  formed  of  a  chain  of 
cells  in  continuity  with  one  another.  Vessels  may  be 
pitted,  scalariform  or  reticulate,  and  have  exactly  the 
same  function  as  chains  of  tracheids.  They  are,  however, 
more  efficient  than  chains  of  tracheids  because,  owing 
to  the  central  perforation  or  complete  disappearance 
of  the  cross  walls,  they  offer  less  resistance  to  the 
passage  of  water.  The  wider  water-conducting  elements 
are  always  vessels.  The  widest  tracheids  are  not  more 
that  100  ju,  in  diameter,  usually  much  less,  while  vessels 
commonly  reach  a  width  of  300  /*,  and  some  measure 
as  much  as  700  p  across. 

Alteration  of  the  Substance  of  the  Cell  Wall.— The 
walls  of  the  living  cells  of  the  plant  body  commonly 
remain  cellulose,  but  in  many  cell  walls  important 
changes  affecting  the  functions  of  the  cell  takes  place 
in  the  wall,  usually  while  the  cell  is  still  alive,  but 
sometimes  after  death. 

One  of  the  most  important  modifications  is  lignification . 
Lignification  occurs  in  its  extreme  form  in  the  walls  of 
the  tracheids,  vessels,  wood  fibres,  and  often  in  the  paren- 
chyma of  the  wood.  It  also  often  affects  other  fibres  to 
a  greater  or  lesser  degree.  It  depends  on  the  deposition 
in  the  wall  of  lignocellulose,  consisting  of  cellulose  and 
two  other  constituents  (an  aromatic  substance  and  a 
pentosan)  often  classed  together  as  lignin.  This  gives 
characteristic  colour  reactions  with  various  chemical 
reagents,  such  as  phloroglucin  and  hydrochloric  acid 
(magenta  red),  pyrogallol  and  hydrochloric  acid  (blue- 
green),  aniline  chloride  or  aniline  sulphate  (golden 


284  TISSUE   ELEMENTS   OF   SEED   PLANTS 

yellow).  Lignified  walls  also  take  up  the  aniline  dyes 
strongly,  so  that  the  walls  of  the  tracheids  and  vessels 
and  often  also  of  the  fibres  are  deeply  coloured  in 
permanent  microscopic  preparations  of  plant  tissues 
that  have  been  stained  with  these  dyes.  Lignification 
hardens  the  wall  considerably,  but  does  not  decrease 
its  permeability  to  water. 

In  contrast  to  lignification,  in  which  lignin  is  laid 
down  in  a  cellulose  matrix,  is  the  formation  of  parts 
of  certain  cell  walls,  not  by  cellulose,  but  by  one  of 
two  aggregate  substances  known  as  cutin  and  suberin. 
These  are  substances  which  differ  chemically  from  one 
another,  but  are  closely  allied  and  have  the  same 
physical  property  of  being  impermeable  to  water. 
They  are  not  true  fats,  but  they  are  allied  to  fats  and 
show  some  of  the  same  reactions,  for  instance  staining 
with  Sudan  3.  Cutin  is  the  substance  of  the  cuticle 
covering  the  epidermis  of  the  shoot,  and  suberin  forms 
one  of  the  layers  of  the  wall  in  cork  cells.  Hence 
both  the  outer  walls  of  the  epidermis  of  the  herbaceous 
shoot  and  also  cork  tissue  (which  is  formed  in  the  bark 
of  woody  plants,  see  Chapter  XX)  are  practically 
impermeable  to  water,  and  thus  prevent  the  drying 
up  of  the  shoot  by  evaporation.  The  impermeability 
of  cork  to  water  accounts  of  course  for  the  use  of  the 
bark  of  the  cork  oak  (Quercus  suber],  which  is  very 
pure  cork,  to  make  bottle  corks. 

The  contrast  between  cutin  and  cellulose  can  be 
demonstrated  by  treating  a  section  through  the  epidermis 
with  Schulze's  solution  (solution  of  iodine  in  zinc 
chloride)  which  stains  the  cellulose  blue,  owing  to  the 
formation  of  "  amyloid,"  a  substance  that  gives,  like 
starch,  a  blue  colour  with  iodine,  but  the  cutin  of  the 
cuticle  yellow. 


MUCILAGE.      INTERCELLULAR   SPACES  285 

Another  important  substance  of  the  cell  wall  is 
pectin,  of  which  the  middle  lamella  (the  original  thin 
wall  formed  at  cell  division)  is  often  composed.  Pectin 
is  also  formed  in  the  cell  walls  (and  cell  contents)  of 
succulent  fruits.  The  pentosans  (see  p.  275),  which, 
as  we  have  seen,  are  characteristic  of  certain  succulent 
plants,  also  occur  in  lignified  cell  walls,  being  the 
chief  constituents  of  the  "  wood  gums."  The  presence 
of  these  and  other  carbohydrates  which  readily  take  up 
water  and  become  mucilaginous  are  the  cause  of  the 
ready  formation  of  slimy  substances  from  many  cell  walls 
and  cell  contents  on  addition  of  water,  for  instance  the 
swelling  of  the  middle  lamellae  of  the  medullary  cells 
of  Fucus,  the  swelling  of  the  surface  cells  of  linseed 
(p.  137),  the  breakdown  into  mucilage  of  the  external 
cells  of  the  root  cap  (p.  290),  and  many  other  cases. 
The  process  is  the  absorption  of  water  by  a  carbo- 
hydrate gel,  quite  comparable  with  the  swelling  of 
gelatine  considered  in  Chapter  III  (p.  53). 

Intercellular  Spaces. — Most  of  the  living  tissues  of 
the  higher  plants  are  interpenetrated  by  a  system  of  air- 
containing  spaces  (intercellular  spaces)  initiated  by  the 
breaking  apart  of  the  cells  along  the  plane  of  the  middle 
lamella.  The  air  in  these  spaces  is  often  called  the 
"  internal  atmosphere  "  of  the  plant.  Its  composition 
agrees  more  or  less  closely  with  ordinary  atmospheric 
air,  but  except  in  tissues  like  the  mesophyll  of  the 
leaf  (Fig.  10),  which  are  constantly  setting  free  oxygen 
as  the  result  of  the  photolysis  of  carbon  dioxide,  it 
contains  a  smaller  proportion  of  oxygen  and  a  larger 
proportion  of  carbon  dioxide  than  atmospheric  air, 
owing  to  the  respiration  of  the  living  cells,  which  use 
up  oxygen  and  set  free  carbon  dioxide.  The  air  in  the 
intercellular  spaces  is  normally  saturated  or  nearly 


286  TISSUE   ELEMENTS   OF   SEED   PLANTS 

saturated  with  water  vapour,  because  it  is  surrounded 
by  living  cells  saturated  with  water.  The  exchange  of 
gases  between  the  living  cells  in  the  interior  of  the 
plant  and  the  external  air  takes  place  by  diffusion 
through  this  system  of  intercellular  spaces. 

PRACTICAL  WORK. 

(1)  Ordinary   living  parenchyma.     Several   examples   of   this, 
with  and  without  chloroplasts,  have  already  been  seen. 

(2)  Secretory  (gland]  cells.     Examine  demonstration  specimens 
of  sections  through  the  enzyme  secreting  glands  on  the  inner 
surface   of   the   "  pitcher "    of    the    Pitcher    Plant    (Nepenthes) 
and  also  of  sections  through  a  nectary.     Note  in  both  cases  that 
the  cells  are  densely  filled  with  cytoplasm  and  have  conspicuous 
nuclei. 

(3)  Thick-walled  cells,   local  thickening  of  cell  walls,  secretory 
cells  and  protein  cells  in  transverse  section  of  leaf  of  pine  (a  per- 
manently stained  and  mounted  section  of  a  large-leaved  species, 
such  as  P.  pinaster,  is  best). 

Examine  first  the  thick-walled  cells  in  the  corner  of  the  leaf. 
Those  just  below  the  surface  layer  have  abundant  cytoplasm 
and  conspicuous  nucleus — they  are  still  active.  Note  the 
stratification  of  the  walls,  the  middle  lamella,  and  the  pits. 
The  cavities  of  the  epidermal  cells  are  almost  obliterated.  Note 
the  thick  layer  of  cuticle  on  their  outer  surfaces. 

The  mesophyll  (photosynthetic)  cells  below  the  thick-walled 
tissue  have  numerous  choloroplasts  and  plate-like  projections 
of  the  walls  (local  thickenings)  into  the  cavities. 

Note  the  resin  canals  at  intervals  (one  in  each  corner  of  the 
leaf).  These  are  intercellular  channels  surrounded  by  secretory 
cells,  and  (outside  these)  thick-walled  cells.  The  resin  is  expelled  by 
the  secretory  cells  and  accumulates  in  the  canal. 

Examine  the  protein  cells  on  the  outer  flanks  of  the  central 
pair  of  bundles.  These  are  rich  in  cytoplasm  and  have  conspicuous 
nuclei.  They  adjoin  the  sieve  tubes  of  the  bundles. 

Note  also  the  lens-shaped  bordered  pits  on  the  walls  of  some 
of  the  cells  around  the  bundles. 

(4)  Stone  cells.     Tease  out  on  a  slide  in  a  drop  of  dilute  glycerine 
a  little  of  the  flesh  of  a  pear,  and  note  the  thick-walled  pitted 
stone  cells,  singly  and  in  groups,  among  the  thin-walled  paren- 
chyma.    When  the  stone  cells  are  numerous  the  pear  is  gritty 
between  the  teeth. 


PRACTICAL   WORK  287 

(5)  Sieve  tubes  and  companion  cells.     Examine  the  demonstra- 
tion preparations  of  sieve  tubes  and  companion  cells  in  trans- 
verse and  longitudinal  section  of  the  stem  of  Cucurbita.     Note 
the  thick  perforated   sieve  plates   (cross   walls   of  sieve   tubes), 
the  callose,  the  cytoplasm  contracted  from  the  side  walls  of  the 
tube  but  not  from  the  plates,   and  the  narrow  companion  cells 
(protein  cells)   with  granular  contents  and  conspicuous  nuclei. 
Measure  with  the  micrometer  eyepiece  the  diameters  of  a  sieve 
tube,  of  a  companion  cell  and  of  the  pores  of  the  sieve  plate. 

(6)  Vessels,  fibres   and   wood-parenchyma.     Examine    a   piece 
of    Cucurbita    stem    and   identify   the    longitudinally    running 
strands  (bundles)  with  large  openings  (vessels)  visible  to  the  naked 
eye.     Tease  a  little  of  a  previously  macerated  bundle  from  the 
stem  of  Cucurbita  in  a  drop  of  dilute  glycerine,  and  observe  the 
narrow  spiral  and  annular  vessels  and  pieces  of  the  broad  reticu- 
late vessels.     Measure  their  diameters.     Thin-walled  parenchyma 
cells  are  associated  with  the  vessels.     Treat  similarly  a  little 
macerated   willow   wood   and   note   that  it   consists   mainly  of 
fibres — long  narrow  cells  with  thick  walls  and  tapering  ends — 
and  of  a  few  vessels  bearing  close-set  bordered  pits  where  they 
abut  on  one  another.     Measure  the  diameter  of  a  vessel.     Note 
also  the  thick-walled  oblong  cells  with  simple  pits  running  through 
the  wood  in  plates  (medullary  rays). 

(7)  Water  tissue.     Note  the  massive  thin-walled  tissue  forming 
the  bulk  of  the  succulent  leaf  of  Kleinia  (or  other ' '  leaf  succulent ' ') . 
Contrast  this  with  the  layer  of  tissue  containing  chloroplasts  near 
the  surface.     The  green  cells  draw  on  the  water  tissue  when 
they  lose  water  more  rapidly  than  it  can  be  supplied  through 
the  bundles  from  the  roots. 

Examine  transverse  sections  of  the  fresh  Begonia  leaf  and  note 
the  many  layers  of  thin-walled  colourless  cells  on  the  upper  side. 
This  is  derived  from  the  upper  epidermis  by  divisions  parallel 
with  the  surface.  These  cells  are  living,  and  the  very  thin  cyto- 
plasmic  lining  of  the  cell  walls  can  be  seen  here  and  there.  The 
water  tissue  on  the  lower  side  contains  chloroplasts,  and  is  not 
derived  from  the  epidermis  but  from  the  mesophyll  tissue.  It 
is,  however,  very  distinct  from  the  main  photosynthetic  layer 
in  the  centre,  which  is  densely  packed  with  chloroplasts. 


CHAPTER   XVII 
THE    ROOT 

HAVING  now  obtained  some  knowledge  of  the  appearance 
and  structure  of  the  elements  of  which  the  tissues  of 
the  bodies  of  the  higher  plants  are  composed,  we  pass 
on  to  consider  the  nature,  structure  and  functions  of  the 
main  vegetative  organs  of  the  plant — the  root  and  the 
shoot  (stem  and  leaves) — aggregations  of  these  tissues. 

Roots  are  colourless  axes  of  the  plant  which 
typically  descend  into  and  grow  in  the  soil,  fixing 
the  plant  and  absorbing  water  and  dissolved  salts 
from  the  soil.  It  is  pretty  certain  that  they  are  to 
be  regarded  as  specialised  underground  branches  of  the 
thallus  which  formed  the  body  of  remote  ancestors  of 
the  higher  plants.  According  to  their  origin  in  the 
life  of  the  individual  seed  plant  we  distinguish  the 
primary  root  system,  i.e.  the  taproot  or  main  descending 
axis  of  the  plant  (already  laid  down  in  the  embryo) 
and  its  branches,  from  the  adventitious  roots  which 
arise  from  some  part  of  the  shoot,  e.g.  from  a  rhizome 
or  from  the  lower  part  of  an  erect  stem  (see  Chapter  XV, 
p.  261).  These  two  kinds  of  roots  do  not,  however, 
differ  structurally  in  any  essential  respect. 

Tropisms  of  Roots. — Some  of  the  most  important 
physiological  characters  of  roots  are  their  distinctive 
tropisms  (see  Chapter  V,  p.  87).  The  taproot  is  in 
the  great  majority  of  cases  positively  geotropic,  i.e.  it 
grows  towards  the  centre  of  the  earth,  and  if  it  is  placed 


TROPISMS  289 

in  a  horizontal  position  the  region  just  behind  the  tip 
bends  so  as  to  bring  the  apex  pointing  vertically  down- 
wards. This  bending  is  carried  out  by  the  cells  on  the 
upper  side  of  the  elongating  region  growing  faster  than 
those  on  the  lower  side,  but  how  the  root  perceives 
the  stimulus  of  gravity  is  not  by  any  means  fully 
understood.  The  branches  of  the  taproot  do  not 
grow  vertically  downwards  but  obliquely,  making  some 
angle  less  than  a  right  angle  with  the  continuation  of 
the  main  root. 

Many  roots  are  also  negatively  phototropic,  i.e.  they 
tend  to  grow  away  from  a  source  of  bright  light.  The 
strongest  tropism  of  roots  is,  however,  positive  hydro- 
tropism,  or  the  tendency  to  grow  towards  a  region  of 
greater  moisture.  In  the  case  of  a  taproot  growing 
in  ordinary  moist  soil  these  three  tropisms  will  clearly 
all  act  in  the  same  direction,  they  will  all  take  the  root 
directly  downwards,  for  not  only  is  this  the  direction 
of  the  pull  of  gravity,  it  is  the  direction  away  from  the 
light,  and  it  is  also  the  direction  of  greater  moisture, 
for  the  surface  layers  of  soil  will  be  drier  than  the  deeper 
ones  as  a  result  of  evaporation  to  the  air.  But  if  the 
soil  is  rather  dry  and  the  source  of  moisture  is  to  one 
side,  then  the  roots  will  bend  in  that  direction,  the 
hydrotropic  tendency  overcoming  the  others. 

Growing  roots  require  a  supply  of  free  oxygen  for 
respiration,  and  the  roots  of  most  ordinary  land  plants 
flourish  best  and  develop  root  hairs  (their  absorbing 
organs)  most  abundantly  in  well  aerated  soil,  in  which  the 
air  is  saturated  or  nearly  saturated  with  water  vapour. 

Structure. — The   apical    (primary)    meristem   of    the 

root  is  covered  by  a  root  cap  (Fig.  47,  A,  r.c.},  thickest 

at  the  apex  and  gradually  thinning  at  the  sides  till  it 

comes  to  an  end  a  short  distance  behind  the  tip.     The 

19 


2QO  THE   ROOT 

root  cap  is  composed  of  living  cells  derived  from  the 
primary  meristem  (p.m.]  from  which  they  are  continually 
renewed  by  active  cell  division,  while  those  on  the  surface 
of  the  cap  break  down  into  mucilage  and  are  rubbed 
off  as  the  root  tip  is  pushed  through  the  soil.  Behind 
the  root  tip  (Fig.  47,  a,  A)  there  is  a  bare  stretch  of 
varying  length,  often  about  5  to  10  mm.  long,  where 
the  root  is  actively  growing  in  length  :  this  is  the 
elongating  region  (E).  Behind  this  again  is  the  root- 
hair  region  (R),  thickly  covered  under  favourable 
conditions  of  soil  moisture  and  aeration,  with  root 
hairs,  each  of  which  is  a  tubular  outgrowth  from  a 
single  cell  of  the  surface  layer  (piliferous  layer]  (Fig.  48). 
The  root  hairs  come  into  intimate  contact  with  moist 
soil  particles  and  absorb  water  owing  to  the  greater 
osmotic  strength  of  the  cell  sap  than  of  the  dilute 
solution  outside,  and  also  salts  in  solution.  It  should 
be  understood  that  the  entrance  of  the  salts  into  the 
root  hair  is  independent  of  the  entrance  of  water,  and 
depends  primarily  on  the  difference  of  concentration  of 
the  various  solutes  inside  and  outside  the  root-hair  cell. 
The  ectoplasm  of  the  root  hairs,  which  is  impermeable 
to  the  "  osmotic  substances "  in  the  cell,  must  of 
course  be  permeable  to  these  salts.  New  root  hairs  are 
formed  in  front  of  the  root  hair  region  by  the  growing 
out  of  the  cells  of  the  piliferous  layer  into  papillae  which 
elongate  to  form  the  hairs  (Fig.  49,  A).  The  hairs 
at  the  back  usually  die,  but  in  some  cases  they  are 
persistent  and  clothe  the  root  for  a  long  distance  even 
to  the  point  of  origin  of  a  root  several  inches  long. 
But  ordinarily  the  life  of  a  root  hair  is  short,  and  behind 
this  region  the  root  is  again  bare. 

Growth  of  the  Root  Tip. — As  new  cells  are  formed  by 
division  of  the  meristem  the  ones  already  formed  pass 


STRUCTURE 


291 


into  the  elongating  region,  increase  actively  in  length 
by  the  formation  and  enlargement  of  the  vacuole,  and 


a 


m 


p 

r.c 


FIG.  47. — Diagrams  illustrating  the  structure  of  the  root,  a,  in 
longitudinal  section.  A,  apical  region  ;  E,  elongating  region  ;  R, 
root-hair  region;  r.c..  root  cap  ',p.m.,  primary  meristem ;  v .c.,  vascular 
cylinder;  px.,  protoxylem;  c,  cortex;  p.l.,  piliferous  layer ;  r.h.,Toot 
hairs,  b,  part  of  vascular  cylinder  (the  conjunctive  tissue  ot 
the  cylinder  is  supposed  to  be  transparent,  the  endodermis  opaque). 
C,  the  same  with  endodermis  removed;  end.,  endodermis;  per., 
pericycle ;  ph.,  phloem ;  px.,  protoxylem ;  mx.,  metaxylem.  d,  Dia- 
gram of  cross-section  of  cylinder  through  origin  of  branch  root ; 
cort,  end/,,  Xg,,  p.mt,  r.Ct,  cortex,  endodermis,  xylem,  primary 
meristem  and  root  cap  of  branch  root.  On  the  left  the  primary 
meristem  of  the  opposite  branch  is  just  formed  in  the  pericycle. 
The  root  in  b — d  is  tetrarch,  i.e.  with  4  xylem  and  4  phloem  strands. 

gradually  become  differentiated  into  the  various  tissues. 
The  force  of  elongation  of  the  cells  in  this  region  exerts 


2Q2 


THE   ROOT 


a  push  up  and  down  the  root.  The  region  behind  the 
elongating  region  is  held  fast  in  the  soil  by  the  root 
hairs,  but  the  root  apex,  covered  by  the  root  cap,  is 


FIG.  48. — Cross-section  of  a  small  root  taken  in  the  root-hair  region  : 
r.h.,  root  hairs;  p. I.,  piliferous  layer;  cor.,  cortex;  e,  endodermis ; 
per.,  pericycle;  px.,  protoxylem;  mx.,  rnetaxylem;  ph,.  phloem  (s.t., 
sieve  tube;  c.c.,  companion  cell).  This  root  is  diarch,  i.e.  with 
2  xylem  strands,  joined  to  form  a  plate,  and  2  phloem  strands. 

pushed  between  the  particles  of  soil,  its  passage  being 
lubricated  by  the  mucilaginous  surface  of  the  cap. 

Internal  Structure. — A  cross  -  section  of  the  root 
taken  in  the  middle  of  the  root-hair  region  gives  the 
best  idea  of  the  primary  plan  of  construction  of  the 
root.  On  the  surface  is  the  piliferous  layer  (Fig.  48,  p. I.}, 


ENDODERMIS   AND    VASCULAR  CYLINDER  2Q3 

each  cell  of  which  is  capable  under  favourable  conditions 
of  growing  out  to  form  a  root  hair  (r.h.}.  Within 
is  the  cortex  (cor.},  composed  of  thin-walled  parenchyma 
interpenetrated  by  intercellular  spaces.  In  the  centre 
is  the  vascular  cylinder,  containing  the  conducting 
(vascular)  system. 

The  vascular  cylinder  is  separated  from  the  cortex 
by  a  single  layer  of  cells,  the  endodermis  I  (Fig.  48,  e). 
The  lateral  and  transverse  cell  walls,  i.e.  those  walls  which 
are  common  to  adjacent  cells  of  this  layer,  or  the  central 
strips  of  these  walls  (Fig.  49,  B),  are  composed  of  cutin 
(see  p.  284)  :  the  outer  and  inner  walls  of  the  endodermis, 
i.e.  those  abutting  on  the  cortex  and  the  tissues  of  the 
cylinder  respectively,  are  cellulose.  Thus  the  endodermis 
is  stiffened  and  united  into  a  more  or  less  rigid  layer 
whose  cells  cannot  be  separated  (Fig.  49,  C),  and  there 
is  a  free  passage  for  w:ater  and  solutes  from  cortex 
to  cylinder  only  through  the  inner  and  outer  walls  and 
through  the  bodies  of  the  endodermal  cells. 

Immediately  below  the  endodermis  is  a  layer  of 
parenchymatous  cells,  the  pericycle  (Fig.  48,  per.}* 
generally  only  one  cell  thick.  Below  and  immediately 
abutting  on  the  pericycle  come  the  vascular  (conducting) 
elements  proper,  arranged  in  alternating  strands  of 
tracheids  and  vessels — xylem  3  and  sieve  tubes  (s.t.) 
with  companion  cells  (c.c.) — phloem.*  The  xylem  and 
phloem  strands  are  separated  from  one  another  by 

1  Greek  evdov,  within,  dep/iia,  skin,  i.e.  the  inner  skin,  the  skin  of 
the  vascular  cylinder.  This  is  not  to  be  verbally  confused,  as  is  often 
done  by  beginners  in  biology,  with  the  "  endoderm,"  the  lining  of 
the  primitive  gut  of  animals.  The  terms  are  both  derived  from  the 
same  Greek  words,  but  have  a  very  different  technical  meaning.  It 
is  therefore  important  to  keep  the  distinctive  ending. 

*  Greek  Tiepi,  around,  and  /cv/cAog,  a  ring,  "  the  cell  layer  surround- 
ing the  cylinder." 

3  Greek  £v\ov,  wood — the  wood  of  a  tree  is  secondary  xylem,  seep.  331. 

4  Greek,  ^Aotdg,  bark — the  inner  bark  of  a  tree  is  secondary  phloem 
(P-  340)- 


294 


THE   ROOT 


FIG.  49. — A,  three  cells  of  the  piliferous  layer  at  the  beginning  of 
the  root-hair  region  in  longitudinal  section,  showing  the  early 
stages  of  the  development  of  root  hairs.  Note  the  accumulation 
of  cytoplasm  and  the  presence  of  the  large  nucleus  (which  passes 
into  the  hair)  at  the  point  of  origin  of  the  root  hair.  B,  diagram 
of  single  endodermal  cell,  showing  the  cutinised  band  (black) 
round  the  radial  and  horizontal  walls.  The  tangential  walls 
(front  and  back)  are  supposed  to  be  transparent.  C,  diagram 
of  portion  of  endodermis,  supposed  to  be  seen  from  within  the 
cylinder,  showing  the  way  the  cells  are  fitted  together  and 
"  cemented  "  by  the  cutinised  bands.  D,  small  portion  of  edge 
of  cylinder  in  an  old  and  large  root,  showing  "  horse-shoe  " 
thickening  of  endodermis,  and  "  passage  cell  "  opposite  proto- 
xylem.  E,  diagram  of  cross-section  of  vascular  cylinder  in 
which  secondary  thickening  has  begun,  the  secondary  tissue 
at  present  formed  only  opposite  the  primary  phloems.  Xt, 
primary  xylem ;  X3,  secondary  xylem  ;  PI,  primary  phlom  ;  C, 
cambium. 


PROTOXYLEM  AND  METAXYLEM         2Q5 

parenchymatous  rays  which  are  continuous  with  the 
pericycle.  In  slender  cylinders,  where  the  number  of 
alternating  strands  of  xylem  and  phloem  is  small  the 
centre  of  the  cylinder  is  filled  up  with  large  xylem 
vessels  in  contact  with  the  strands  of  xylem  which 
abut  on  the  pericycle  (Fig.  48).  In  massive  cylinders 
where  the  number  of  xylem  and  phloem  strands  is 
large  the  centre  is  occupied  by  parenchyma  (pith). 

The  outermost  tracheids,  abutting  on  the  pericycle, 
are  narrow,  and  spiral  or  annular.  They  are  the  first 
xylem  elements  to  be  formed,  and  are  hence  called 
protoxylem  (Fig.  48,  px.}.  Within  these  are  larger  pitted 
vessels  (metaxylem)  (Fig.  48,  mx.),  which,  in  a  narrow 
cylinder,  fill  up  the  centre  (Fig.  48).  The  spiral  or 
annular  thickenings  of  the  outermost  tracheids  are 
laid  down  and  lignified  in  the  elongating  region  of  the 
root,  the  protoplasm  dying  after  the  thickenings  are 
complete.  The  thin  cellulose  wall  of  the  tracheid  is 
passively  stretched  by  the  active  elongation  of  the 
surrounding  parenchymatous  tissue,  and  the  successive 
turns  of  the  spiral  thickening  (or  the  annuli)  are  pulled 
apart  (Fig.  45,  B — G).  As  long  as  they  are  not  too 
widely  separated,  the  internal  thickenings  of  the  tracheid 
prevent  the  obliteration  of  the  cavity  by  the  pressure 
of  the  surrounding  turgid  tissue,  and  so  this  first 
formed  water-conducting  channel  is  kept  open. 

Thus  the  spiral  and  annular  tracheids  are  very 
beautifully  adapted  to  perform  their  function  under 
the  conditions  in  which  they  are  formed,  for  they  both 
admit  of  longitudinal  extension  to  keep  pace  with 
the  growth  in  length  of  the  tissue,  and  are  protected 
against  obliteration  by  the  surrounding  turgid  tissue. 
In  organs  which  elongate  a  great  deal,  the  spiral 
tracheids  first  formed  are  ultimately  stretched  so  much 


296  THE   ROOT 

that  the  thickening  no  longer  supports  the  wall  and 
the  tracheids  are  crushed  (Fig.  45,  F). 

A  transverse  section  across  the  elongating  region 
shows  the  protoxylem  elements  already  lignified..  but 
the  cells  which  will  develop  into  metaxylem  vessels 
still  thin  walled  and  living.  In  the  root-hair  region 
the  walls  of  these  gradually  thicken  and  lignify,  and 
various  stages  in  their  development  can  be  traced. 

Absorptive  Function  of  the  Root.  —Owing  to  the 
existence  of  substances  such  as  sugar,  which  attract 
water,  in  the  cell  sap  of  the  root  hair,  water  is  drawn 
in  from  the  soil  outside.  As  the  solution  within  the 
vacuole  becomes  more  dilute,  the  absorptive  power  of 
the  root-hair  cell  decreases,  and  when  the  dilution 
has  passed  a  certain  point  the  absorptive  power  of  the 
adjoining  cortical  cells,  whose  cell  sap  will  then  be  more 
concentrated  than  that  of  the  root-hair  cell,  will  cause 
water  to  be  drawn  into  them  from  the  root-hair  cell. 
In  the  same  way  water  will  be  drawn  across  the  cortex 
from  cell  to  cell  through  the  endodermis  and  pericycle 
to  the  xylem  tracheids  and  vessels.  The  mechanism 
of  the  passage  of  water  from  the  living  cells  of  the 
cylinder  into  the  xylem  tracheids  and  vessels  is  not 
fully  understood,  but  it  seems  probable  that  osmotically 
active  substances  exist  in  considerable  quantities  in 
these  also.  When  they  lose  their  protoplasm  after 
differentiation  a  considerable  amount  of  organic 
substance  must  be  left  in  their  cavities. 

It  must  be  clearly  understood  that  the  passage  of 
solutes,  for  instance  the  mineral  salts — nitrates,  sulphates 
and  phosphates — absorbed  from  the  soil  and  ultimately 
passing  into  the  xylem,  do  not  necessarily  travel  across 
the  cortex  at  the  same  rate  as  the  water.  Their  passage 
depends  on  the  state  of  equilibrium  between  the  different 


BRANCHING  297 

cells  in  regard  to  each  salt,  and  on  the  relative  perme- 
ability of  the  cytoplasmic  membranes  to  the  different 
salts. 

Structure  of  the  Older  Parts  of  the  Root.— In  ex- 
ceptional cases,  as  has  already  been  said,  root  hairs 
may  persist  indefinitely  during  the  life  of  the  root, 
but  normally  their  life  is  quite  short,  and  they  die 
and  peel  off,  with  the  piliferous  layer,  behind  the 
limited  root-hair  region.  The  surface  of  the  root  is 
then  formed  by  the  outer  cells  of  the  cortex,  and  the 
outer  walls  of  these  cells,  now  in  contact  with  the  soil, 
are  cutinised  and  no  longer  absorb  water  from  the  soil. 
Meanwhile  the  metaxylem  vessels  of  the  vascular 
cylinder  are  now  completely  lignified,  and  the  conduct- 
ing capacity  of  the  primary  structure  has  reached  its 
fullest  extent.  Secondary  changes  may  now  occur  : — 

(i)  Branching. — Branch  roots  arise  in  this  region,  by 
the  division  of  the  cells  of  the  pericycle,  usually  opposite 
a  protoxylem  strand.  A  new  apical  meristem  is 
established,  covered  by  a  root  cap,  and  the  tip  of 
the  new  root  grows  out  through  the  endodermis  and 
cortex  of  the  mother  root  (Fig.  47,  d),  partly  by  digesting 
the  cells,  partly  by  mechanical  pressure,  and  emerges 
into  the  soil.  When  a  root  is  branching  freely  a 
fairly  close-set  row  of  branches  may  grow  out  from 
opposite  each  protoxylem  of  the  mother  root,  so  that 
if  the  mother  root  is  tetrarch,  four  such  rows  of  branches 
will  appear.  The  vascular  tissues  of  the  new  root 
establish  connexions  with  the  corresponding  tissues  of 
the  mother  root  ;  thus  the  xylem  strands  connect 
with  the  xylem  opposite  which  the  new  root  has  arisen 
by  the  differentiation  of  tracheids  in  the  intervening 
tissue,  the  phloems  by  the  formation  of  sieve  tubes 
connecting  with  the  sieve  tubes  of  the  two  phloem 


298  THE   ROOT 

strands  on  each  side  of  the  point  of  origin.  In  this 
way  the  conducting  systems  of  the  mother  root  and  of 
the  branch  roots  become  part  of  one  system.  The 
cortex  of  the  branch,  however,  never  has  any  con- 
nexion with  the  cortex  of  the  mother  root. 

(2)  Secondary  Thickening. — In  perennial  plants  which 
have  a  persistent  primary  root  system,  and  also  in 
many  annuals,  the  primary  tissues  are  added  to  by 
the  activity  of  a  secondary  meristem.  In  other  words, 
certain  living  cells  of  the  primary  tissue  begin  to  divide, 
and  the  products  of  their  division  become  differentiated 
to  form  new  permanent  tissues,  which  are  called 
secondary  tissues,  especially  new  (secondary)  xylem  and 
phloem.  This  process  takes  place  in  the  roots  and 
stems  of  all  the  woody  plants,  and  to  a  certain  extent 
in  most  herbaceous  plants  belonging  to  the  Dicoty- 
ledons (the  largest  group  of  seed  plants)  and  to  the 
Conifers  (pines,  firs,  etc.).  The  secondary  vascular 
meristem  is  called  the  cambium,  and  its  activity  results 
in  the  great  and  continual  increase  in  thickness  which 
occurs  in  the  stems  and  roots  of  trees.  We  shall  have 
to  consider  it  again  in  connexion  with  the  woody  stem, 
but  we  may  here  note  the  beginning  of  the  process 
in  persistent  roots. 

The  formation  of  the  cambium  starts  by  the  tangential 
division  of  the  conjunctive  cells  just  inside  and  on  the 
flanks  of  the  primary  phloem  strands.  The  cells  cut 
off  on  the  inside,  i.e.  towards  the  centre  of  the  root, 
become  new  (secondary)  xylem,  the  strands  so  formed 
alternating  with  the  primary  xylem  strands  (Fig.  49,  E). 
A  few  sieve  tubes  are  cut  off  on  the  outside  of  the 
cambium  and  added  to  the  primary  phloem.  The 
cambium  now  extends  round  the  outside  of  the  primary 
xylem  strands  so  as  to  form  a  complete  layer  round 


SECONDARY   TISSUES.      CORK   FORMATION  2-Q9 

the  cylinder.  This  layer  forms  a  wavy  line  as  seen 
in  cross  section,  sweeping  out  round  the  primary 
xylem  and  in  round  each  primary  phloem  (Fig.  49,  E,  c). 
Opposite  the  primary  xylems  it  sometimes  forms  nothing 
but  parenchymatous  tissue  (secondary  rays),  but  in 
other  species  it  forms  xylem  to  the  inside  and  phloem 
to  the  outside,  just  as  it  does  opposite  the  primary 
phloems.  Owing  to  its  greater  activity  at  first  opposite 
the  primary  phloems  and  to  the  formation  of  hard 
masses  of  secondary  xylem  in  these  regions  the  cambium 
soon  straightens  out  and  becomes  circular  in  cross- 
section.  In  the  root  of  a  tree  it  continues  its  activity 
year  by  year,  and  eventually  forms  a  bulky  cylinder 
of  secondary  wood,  and  a  much  thinner  cylinder  of 
secondary  phloem. 

(3)  Cork  Formation. — Another  secondary  meristem 
besides  the  cambium  also  arises  in  woody  roots,  usually 
in  the  pericycle.  This  is  called  the  cork  cambium  or 
phellogen.  because  it  cuts  off  cells  to  the  outside,  certain 
layers  of  whose  walls  are  formed  of  suberin.  These  are 
called  cork  cells,  and  owing  to  the  impermeability  of  the 
corky  walls  to  water  and  solutes,  the  cortex  and  endo- 
dermis  are  cut  off  from  any  functional  connexion  with 
the  vascular  cylinder  and  soon  scale  off.  This  formation 
of  "  bark  "  on  woody  roots  is  quite  parallel  to  that  which 
takes  place  on  tree-trunks.  On  the  inside  the  phellogen 
often  forms  parenchymatous  tissue,  which  is  sometimes 
called  the  "  secondary  cortex." 

Modified  Boots. — In  some  species  the  roots,  or  some 
of  them,  depart  more  or  less  from  the  typical  root 
structure  and  behaviour.  In  some  plants  (climbers 
and  epiphytes)  they  may  turn  green  and  carry  on 
photosynthesis  for  the  plant.  Later  they  may  grow 
down  to  and  enter  the  soil,  then  assuming  typical 


300  THE   ROOT 

root  characters.  In  parasitic  seed  plants  organs  called 
haustoria  of  very  variable  structure  bore  into  the  tissues 
of  the  host  and  absorb  food  from  it.  Sometimes  these 
haustoria  (Thesium,  Rhinanthus)  are  just  modified 
branches  of  ordinary  soil  roots,  which  fasten  on  and  bore 
into  the  roots  of  other  plants.  In  other  cases  (Mistletoe) 
the  parasite  never  has  any  connexion  with  the  soil  at 
all,  but  the  seed  germinates  on  a  tree,  and  its  root- 
like  haustoria  bore  into  the  tissues  of  the  tree,  obtaining 
water  and  mineral  salts  from  the  xylem.  In  other 
cases,  again  (dodder),  the  haustoria  are  strands  of  tissue 
(in  which  sieve  tubes  may  be  developed)  that  tap 
the  sieve  tubes  of  the  host.  And  finally,  in  the  most 
extreme  cases  (Rafflesia,  a  tropical  parasite),  the  whole 
body  of  the  parasite  consists  simply  of  a  branching 
mass  of  strands  of  tissue,  like  the  hyphse  of  a  fungus, 
which  live  entirely  in  the  tissues  of  the  host,  occasionally 
coming  to  the  surface  and  producing  immense  flowers. 
In  some  plants  (carrot,  turnip,  parsnip)  the  main 
taproot  is  greatly  swollen,  and  acts  as  a  food  storage 
organ  through  the  winter  in  the  same  way  as  a  corm 
or  tuber.  The  great  mass  of  fleshy  tissue  is  produced 
by  the  cambium,  and  may  be  reckoned  as  parenchy- 
matous  secondary  xylem  and  phloem,  in  which  a  few 
strands  of  conducting  elements  are  formed  here  and 
there. 

PRACTICAL   WORK. 

(1)  Draw  a  single  mustard  seedling  that  has  been  germinated 
in  moist  air  (being  careful  to  keep  the  root  moist),  and  note 
the  long  densely  crowded  root  hairs,  not  yet  full  grown  towards 
the  tip  of  the  root,  and  the  bare  elongating  region  between  the 
root   hairs    and    the    apex.     Distinguish    root    from   hypocotyl. 
Compare  with  seedlings  grown  in  soil,  and  in  the  latter  note  the 
clinging  of  the  root  hairs  to  soil  particles. 

(2)  Transfer  a  seedling  to  a  slide,  just  covering  the  end  of  the 
root  with  water  under  a  coverslip,  and  examine  under  the  micro- 


PRACTICAL   WORK  3OI 

scope.  Note  especially  the  region  of  short  (young)  root  hairs, 
each  of  which  arises  from  a  surface  cell.  Note  also  the  region 
of  primary  meristem  covered  by  the  root  cap,  the  bare  elongating 
region  and  the  vascular  cylinder  seen  through  the  semi-transparent 
cortex. 

(3)  Examine   the    fresh   taproot,    2    or    3   inches    long,    of    a 
Bean    seedling.     Observe    the    root   cap   covering   the    growing 
point  and  the  four  or  more  longitudinal  rows  of  lateral  roots. 
Cut  the  root  longitudinally  through  the  centre  in  the  region  of 
origin  of  the  branch  roots,  and  make  out  with  a  lens  that  these 
arise  from  the  surface  of  the  central  cylinder. 

In  the  older  part  of  the  root  the  cortex  can  generally  be  separ- 
ated from  the  cylinder  by  twisting  the  root  so  as  to  break  the 
cortex  and  then  pulling  it  off. 

(4)  Make  a  low  power  diagram  of  a  transverse  section  of  a 
buttercup  root  taken  through  the  root  hair  region,  and  showing 
piliferous  layer,  cortex  and  cylinder,  and  in  the  cylinder  the  cross- 
shaped  xylem  with  four  arms,  the  ends  of  which  are  formed  by 
the  narrow  tracheids  of  the  protoxylem.     Between  the  arms  of 
the  xylem  are  the  four  strands  of  phloem.     Put  in  the  boundaries 
of  the  tissue  regions,  but  not  individual  cells. 

With  the  high  power  examine  the  different  tissues  in  detail, 
and  draw  a  portion  of  the  cylinder,  including  a  segment  of  endo- 
dertnis  and  pericycle,  protoxylem,  large  vessels  of  the  metaxylem  (in 
the  centre  these  still  have  thin  walls — cytoplasm  and  nucleus 
can  sometimes  be  seen  in  them),  also  sieve  tubes  and  companion 
cells  of  the  phloem,  and  conjunctive  tissue.  Starch  grains  can 
be  seen  in  the  cortex. 

(5)  Examine  the  cross-section  of  a  young  Bean  root,  noting 
that  the  outline  of  the  cylinder  is  not  so  easy  to  define  as  in  the 
Buttercup,  the  endodermis  being  less  sharply  distinguished  from 
neighbouring  cells,   but  on  careful  examination  with  the  high 
power  the  cutin  strips  of  the  radial  walls  can    be  made  out. 
The  xylem  strands  are  not  joined  in  the  centre,  which  is  occupied 
by  pith .     The  thicker  walled  cells  forming  the  bulk  of  the  phloem 
are  fibres  :    a  few  sieve  tubes  can  be  seen  outside  these.     In 
older  sections  cambium  (formed  early  here)  and  secondary  tissues 
can  be  made  out. 

(6)  Examine  longitudinal  sections  through  the  centre  of  the 
tips  of  Maize  and  Bean  roots.     Note  in  each  root  cap,  primary 
meristem,  and  the  growth  in  size  and  elongation  of  the  cells  as  the 
apex  is  left.     In  the  Maize  especially  note  the  great  increase 
in  width  of  the  rows  of  cells  which  will  form  the  large  vessels, 
and  the  stages  of  development  of  these. 


CHAPTER   XVIII 

THE   FOLIAGE   LEAF 

THE  leaves  and  stems  of  the  higher  plants  are 
differentiated  organs  of  the  shoot.  In  most  of  the 
lower  chlorophyll-containing  plants  (Fucus,  Pellia)  the 
shoot  does  not  show  this  differentiation,  but  it  is 
established  in  all  the  vascular  plants,  except  a  few 
peculiar  forms  which  have  lost  it.  The  foliage  leaf  is 
essentially  the  organ  of  photosynthesis,  its  principal 
tissue,  the  mesophyll,  containing  the  great  majority  of 
the  chloroplasts  of  the  plant,  though  the  parenchy- 
matous  tissue  of  the  stem  generally  contain  a  certain 
number. 

Essential  Structure.— The  typical  foliage  leaf  (Fig.  50) 
is  essentially  a  thin  plate  of  mesophyll,  thin  enough  to 
allow  light  of  sufficient  intensity  to  reach  all  the  cells, 
with  air  spaces  between  the  cells,  communicating  with 
the  outer  air,  and  thus  permitting  free  diffusion  of  gases 
between  the  cells  and  the  air.  The  mesophyll  is  inter- 
penetrated by  vascular  bundles  (veins)  which  carry  water 
and  salts  coming  from  the  root  (through  the  vessels  and 
tracheids  of  the  xylem)  to  the  mesophyll,  and  formed 
organic  substances  away  from  it  (through  the  sieve  tubes 
of  the  phloem)  to  other  regions  of  the  plant,  principally 
to  growing  regions  and  storage  organs.  The  mesophyll 
is  protected  by  a  layer  of  cells  (the  epidermis],  typically 
not  containing  chlorophyll  and  acting  as  a  water  tissue 
(see  p.  275),  and  the  continuous  outer  layer  of  the 


CUTICLE   AND    STOMATA 


303 


external  walls  of  these  cells,  in  contact  with  the  outer 
air,  consists  of  a  waterproof  covering,  the  cuticle,  com- 
posed of  cutin  (see  p.  284).  If  the  cuticle  were  perfectly 
continuous  over  the  whole  surface  of  the  leaf  it  would 
prevent  the  interchange  of  gases  between  the  mesophyll 
and  the  outer  air,  for  instance  the  supply  of  carbon 
dioxide  which  the  chloroplasts  use  as  part  of  the  raw 


FIG.  50. — Part  of  transverse  section  of  dorsiventral  foliage  leaf  :  cut., 
cuticle ;  u.ep. .upper  epidermis ;  mea., mesophyll ;  pal.,  palisade  tissue  ; 
sp.,  spongy  tissue  ;  int.,  intercellular  space  ,  v.b.,  vascular  bundle 
(in  longitudinal  section,  passing  out  of  the  plane  of  section  to 
the  right — further  to  the  right  a  bundle  is  seen  cut  in  transverse 
section)  ;  sh.,  bundle  sheath;  x,  xylem;  ph.,  phloem  ;  l.ep.,  lower 
epidermis ;  st.,  stoma.  The  black  arrows  indicate  the  passage 
of  liquid  water  from  the  xylem  tracheids  through  the  sheath 
into  the  mesophyll  cells  and  epidermal  cells.  The  white  arrows 
the  evaporation  of  water  from  the  mesophyll  cells  into  the  inter- 
cellular spaces  and  diffusion  out  through  the  stomata. 

material  of  photosynthesis.  The  epidermis,  with  its 
cuticle,  is  in  fact  pierced  by  minute  holes  (stomata), 
each  enclosed  by  two  specialised  epidermal  cells  (guard 
cells),  and  these  pores  lead  into  the  system  of  intercellular 
space  of  the  mesophyll  (Fig.  50). 

Transpiration. — The  pores   of  the  stomata  not  only 
allow  of  the  diffusion  of  carbon  dioxide  from  the  air 


304  THE   FOLIAGE   LEAF 

into  the  leaf  as  it  is  used  up  by  the  chloroplasts  of  the 
mesophyll  cells,  but  also,  of  course,  allow  the  water 
vapour  evaporated  from  the  saturated  mesophyll  cells 
into  the  intercellular  spaces  to  diffuse  out  to  the 
(usually)  drier  air  outside  the  leaf.  This  process  is 
called  transpiration. 

Large  quantities  of  water  are  lost  by  the  plant  in 
this  way.  If  a  plant  growing  in  a  flower-pot  is  weighed, 
and  after  a  certain  interval  weighed  again,  the  loss  in 
weight  represents  almost  entirely  transpired  water,  for 
the  gain  or  loss  in  weight  from  photosynthesis  or  respira- 
tion is  negligible  by  comparison  with  the  loss  by  trans- 
piration. The  pot  itself  and  the  soil  must  be  carefully 
covered  with  a  metal  or  rubber  covering  waxed  to  the 
stem  round  the  hole  through  which  the  stem  passes,  to 
prevent  loss  of  water  by  direct  evaporation  from  pot 
and  soil.  If  desired,  transpiration  from  the  surface  of 
the  stem  through  the  stomata  it  bears  can  also  be 
prevented  by  waxing  its  surface.  In  this  way  it  can  be 
shown  that  it  is  through  the  leaves  that  the  loss  of  water 
in  transpiration  mainly  occurs.  A  sunflower  plant  whose 
total  leaf  area  was  5,616  square  inches  lost  i|-  pints 
of  water  in  12  hours  ;  and  it  has  been  calculated  that 
the  trees  in  an  acre  of  beech  forest  transpire  about 
1,400  tons  of  water  during  the  summer. 

This  constant  loss  of  water  from  the  leaf  is  probably 
useful  to  the  plant  because  it  helps  to  set  up  a  current 
(transpiration  current]  through  the  xylem  vessels  and 
thus  to  bring  up  dissolved  salts  from  the  root  more 
rapidly.  That  active  transpiration  is  not  necessary  to 
the  plant,  or  at  least  not  to  some  plants,  is  shown  by 
the  fact  that  growth  continues  in  tobacco  plants,  for 
instance,  which  are  kept  in  air  saturated  with  water 
vapour,  so  that  transpiration  takes  place  very  slowly 


STRUCTURE   OF   STOMATA  305 

indeed.  Evaporation  from  the  mesophyll  cells  does 
however,  take  place  even  in  saturated  air,  because  the 
living  cells,  owing  to  the  process  of  respiration,  which 
is  always  going  on  and  which  liberates  heat,  are  kept 
at  a  somewhat  higher  temperature  than  the  surrounding 
air. 

Structure  and  Mechanism  of  Stomata. — We  have  seen 
that  if  the  leaf  were  covered  by  a  perfectly  continuous 
layer  of  cuticle  it  would  lose  little  or  no  water,  but  it 
could  not  obtain  carbon  dioxide  from  the  air.  The 
stomata  allow  of  the  carbon  dioxide  getting  in,  but  they 
also  allow  of  water  vapour  getting  out,  and  though  this 
may  be  useful  to  the  plant  it  introduces  the  danger  of 
water  loss  so  rapid  that  it  cannot  be  covered  by  a 
corresponding  supply  from  the  root.  This  danger  is 
partly  met  by  the  automatic  closing  of  the  stomatal 
pore  when  there  is  a  deficiency  of  water  in  the  tissues  of 
the  leaf.  The  guard  cells  of  the  stoma  which  surround 
the  pore,  unlike  the  other  epidermal  cells,  contain 
chloroplasts  (Fig.  51,  C,  E),  and  are  thus  able  to  make 
sugar  and  maintain  a  high  osmotic  pressure  and  power 
of  absorbing  water.  The  guard  cells  are  firmly  joined 
to  one  another  at  the  two  ends,  but  in  the  centre  there 
is  a  split  (the  stomatal  pore)  between  them,  while  the 
side  walls  away  from  the  pore  abut  on,  and  can  be 
pushed  into  the  cavities  of,  the  adjacent  epidermal  cells. 
The  cavity  of  the  guard  cells  is  often  confined  to  the 
middle  part  of  the  cell  as  seen  in  vertical  section  :  above 
(next  the  outer  air)  and  below  (next  the  intercellular 
space  into  which  the  pore  leads)  the  wall  is  thick  and 
cutinised. 

When  the  guard  cells  become  turgid  the  cell  cavities 
increase  in  size,  and  since  the  ends  and  the  top  and 
bottom  are  firmly  held,  the  side  walls  increase  in  length, 
20 


THE   FOLIAGE   LEAF 


306 

the   outer   ones   bulging  into   the   adjacent   epidermal 
cells,  the  pressure  of  whose  cell  sap  is  lower,  and  the 


FIG.  51. — A,  diagram  of  stoma  seen  in  vertical  section.  The  thin 
lines  represent  the  position  of  the  walls  of  the  guard  cells  when 
the  stoma  is  closed,  the  thick  lines  when  it  is  open,  the  outer 
lateral  walls  bulging  into  the  adjacent  epidermal  cells.  B,  the 
same  in  surface  view  (half  the  stoma  only  shown).  C,  stoma 
of  Iris  leaf  in  vertical  section,  showing  the  guard  cells  sunk  below 
the  surface  of  the  epidermis  at  the  bottom  of  the  vestibule  (v). 
D,  the  same  in  surface  view  :  epidermal  cells  focussed,  the 
outlines  of  the  guard  cells  seen  faintly  below.  E,  the  guard 
cells  focussed.  F,  ending  of  a  fine  vein  in  the  mesophyll :  one 
line  of  tracheids  surrounded  by  sheath  cells  with  few  chloroplasts. 


MECHANISM   OF   STOMATA  307 

inner  ones  curving  away  from  each  other  and  thus  opening 
the  pore  (Fig.  51,  A,  B,  heavy  lines).  This  is  the  normal 
condition  in  bright  light  and  when  there  is  plenty  of 
water  in  the  leaf.  If  the  air  outside  becomes  very  dry 
water  vapour  diffuses  out  through  the  pores  very  rapidly, 
and  thus  the  vapour  pressure  in  the  intercellular  spaces 
of  the  mesophyll  is  reduced.  This  in  turn  increases 
the  rate  of  evaporation  from  the  mesophyll  cells  into 
the  intercellular  spaces  and  depletes  the  cells  of  water. 
Water  is  drawn  into  the  mesophyll  cells  from  the 
epidermal  cells  and  eventually  into  these  from  the  guard 
cells  of  the  stomata.  The  guard  cells  in  consequence 
lose  their  turgor  and  become  flaccid,  closing  the  pore  by 
the  straightening  of  the  extensible  portion,  of  the  walls 
(Fig.  51,  A,  B,  thin  lines).1  At  night  also,  owing  to  the 
cessation  of  sugar  formation  by  the  chloroplasts  of  the 
guard  cells,  the  osmotic  pressure  falls  and  the  pore  closes. 
This  automatic  mechanism  closing  the  stomata  when 
the  supply  of  water  is  depleted  does  not  always  completely 
stop  the  loss  of  water  from  the  plant,  as  may  be  seen 
towards  the  close  of  a  hot  dry  day  in  summer,  when  the 
leaves  of  some  plants  become  limp  and  droop  owing  to 
the  continued  loss  of  water  to  the  air,  a  loss  which  they 
are  unable  to  make  good  from  the  root  quickly  enough 
to  maintain  the  turgor  of  the  cells.  This  may  be  due 
partly  to  the  incomplete  closure  of  the  pores,  and  partly 
to  the  fact  that  the  cuticle  is  not  completely  impermeable 
to  diffusion  of  water  vapour.  A  drooping  plant  will 
gradually  recover  if  a  bell  jar  be  placed  over  it.  The  jar 
prevents  the  removal  by  currents  of  air  of  the  vapour 

1  This  mechanism  can  be  imitated  by  cutting  off  a  piece  of  the  inner 
tubing  of  a  bicycle  tyre  10  inches  long  (5  inches  on  each  side  of  the 
valve),  tying  up  the  two  ends  tightly  and  fixing  them  to  a  straight 
piece  of  wood  so  that  the  concave  side  of  the  tube  lies  along  the  wood. 
If  air  is  pumped  into  the  valve  the  tube  curves  away  from  the  wood 
as  it  becomes  inflated,  and  straightens  as  it  is  deflated  again. 


308  THE   FOLIAGE   LEAF 

escaping  from  the  leaf,  and  the  accumulation  of  this 
round  the  leaves  checks  further  diffusion  from  their 
intercellular  spaces,  and  thus  enables  the  turgor  of  the 
leaf  cells  to  be  recovered  by  the  retention  in  the  leaf 
of  the  water  coming  from  the  root. 

Most  land  plants  possess  structures  which  tend  to 
produce  the  same  sort  of  effect  as  the  bell  jar  placed  over 
the  plant,  i.e.  the  prevention  of  the  rapid  removal  by 
air  currents  of  the  water  vapour  escaping  through  the 
stomata.  The  commonest  of  these  is  the  sinking  of  the 
stoma  in  a  pit  (vestibule),  the  bottom  of  which,  occupied 
by  the  guard  cells  and  pore,  is  sunk  below  the  general 
level  of  the  cuticle  (Fig.  51,  C).  Sometimes  there  is  a 
deep  groove  or  cavity  in  the  leaf  containing  several 
stomata.  In  other  cases  the  leaf  rolls  or  folds  up  in  dry 
weather,  thus  protecting  the  surface  bearing  the  stomata. 
In  other  cases,  again,  there  is  a  thick  covering  of  hairs 
either  over  the  whole  surface  bearing  stomata  or  confined 
to  the  cavities  in  which  the  stomata  are  sunk.  All 
these  structures  serve  the  same  purpose — to  keep  the 
air  just  outside  the  stomata  relatively  still  and  thus 
prevent  the  rapid  removal  of  water  vapour  by  currents 
of  air,  which  is  a  very  much  quicker  process  than  the 
slow  diffusion,  which  continues  in  any  case.  The  drying 
effect  of  wind  is  well  known,  and  the  shoots  of  plants 
exposed  to  strong,  and  especially  of  course  to  dry, 
winds,  very  quickly  wilt  and  even  die  (through  loss  of 
water  greater  than  they  can  recover) ,  unless  they  are  very 
well  protected  in  one  of  the  ways  described.  It  is 
among  plants  living  in  very  dry  climates,  or  frequently 
exposed  to  strong  winds  during  the  growing  season,  or 
growing  in  dry  soil,  that  the  structures  described  are 
most  developed,  for  it  is  only  such  plants  (xerophilous  I 

1  Greek  £ t]po^,  dry ;   <f>t\oz,  friend. 


STRUCTURE  OF  MESOPHYLL  309 

or  xeromorphic  plants),  thoroughly  well  protected  against 
water  loss,  that  can  survive  in  these  habitats. 

Structure  of  the  Mesophyll. — The  commonest  type 
of  foliage  leaf  is  the  dorsiventral  leaf,  in  which  the  upper 
and  lower  sides  are  distinctly  differentiated.  In  the 
first  place  the  stomata  are  usually  present  in  much 
greater  numbers  on,  or  are  even  confined  to,  the  lower 
surface  of  dorsiventral  leaves.  Secondly,  the  mesophyll 
is  differentiated  into  two  strata — the  palisade  and  the 
spongy  tissue.  The  palisade  tissue  lies  immediately 
below  the  upper  epidermis.1  It  consists  of  from  one  to 
several  layers  of  cylindrical  or  prism-shaped  cells  with 
their  long  axes  perpendicular  to  the  surface  of  the  leaf, 
and  with  narrow  intercellular  spaces  between  them.3 
The  cytoplasm  lining  the  side  walls  of  the  palisade  cells 
is  packed  with  chloroplasts  (Figs,  n,  50),  the  palisade 
tissue  being  the  main  photosynthetic  tissue  of  the  leaf. 
The  spongy  tissue  occupies  the  space  between  the  palisade 
and  the  lower  epidermis,  and  consists  of  rounded  or 
irregularly  shaped  cells  with  large  intercellular  spaces 
between  them.  Under  each  stoma  there  is  usually  a 
particularly  large  intercellular  space.  The  spongy  cells 
contain  chloroplasts,  but  not  nearly  so  many  as  the 
palisade  cells.  The  spongy  tissue  is  the  main  transpiring 
tissue  of  the  leaf. 

The  more  a  plant  is  exposed  to  sun  and  dry  air  the 
greater  the  development  of  palisade  tissue,  and  in  some 
plants  which  live  in  dry  sunny  climates  the  whole  of  the 

1  Sometimes  separated  from  it  by  a  layer  of  hypoderm  (vird,  below, 
dep/ua,  skin),  which  often  acts  as  a  supplementary  water  tissue,  and 
is  often  thick  walled  and  helps  to  strengthen  the  mechanical  structure 
of  the  leaf. 

1  The  palisade  cells  are  generally  in  close  lateral  contact,  so  that 
on  a  cross-section  of  the  leaf  no  intercellular  spaces  may  be  visible. 
But  if  a  "  flat  "  section  parallel  to  the  leaf  surface  be  made  through 
this  tissue,  the  narrow  intercellular  spaces  can  always  be  seen  between 
the  cells. 


310  THE   FOLIAGE   LEAF 

mesophyll  is  palisade,  thus  reducing  the  rate  of  transpira- 
tion. Leaves  held  on  the  plant  in  a  position  in  which 
both  sides  are  equally  illuminated  frequently  have  the 
two  surfaces  alike,  palisade  being  developed  under  each 
epidermis.  Inversely,  in  plants  growing  in  damp  and 
shady  situations  the  palisade  tissue  takes  on  the 
characters  of  spongy  tissue,  the  cells  are  short  and  are 
separated  by  large  intercellular  spaces  ;  and  in  extreme 
"  shade  leaves  "  the  whole  mesophyll  is  of  the  spongy 
type.  The  deeper  the  shade  and  the  damper  air 
(conditions  generally  found  together  in  nature)  the 
greater  the  total  volume  of  intercellular  spaces  in 
proportion  to  the  total  volume  of  the  mesophyll  cells. 

Water  Tissue. — The  epidermis  of  a  typical  foliage  leaf 
acts,  as  we  have  already  remarked  (p.  275),  as  a  water 
store  for  the  mesophyll  cells  ;  and,  as  we  have  also  seen 
(p.  276),  some  leaves  have  a  many  layered  epidermis 
which  greatly  increases  this  water  store.  In  extreme 
cases  the  epidermal  water  tissue  may  form  the  greater 
part  of  the  thickness  of  the  leaf.  Part  of  the  mesophyll 
may  also  be  very  poor  in,  or  even  destitute  of,  chloro- 
plasts,  e.g.  the  central  tissue  of  isolateral  leaves,  and  this 
also  acts  as  a  water  store  for  the  photosynthetic  tissue. 

Vascular  Bundles  (Veins). — Water  and  salts  are  brought 
to  the  mesophyll  of  the  leaf  through  the  vessels  and 
tracheids  of  the  xylem ;  while  sugars  and  organic 
nitrogenous  substances  are  conducted  away  from  the 
mesophyll  through  the  sieve  tubes  of  the  phloem.  A 
strand  of  xylem  and  a  strand  of  phloem  are  associated 
together  to  form  a  vascular  bundle,  the  xylem  being 
towards  the  upper,  the  phloem  towards  the  lower  face 
of  the  leaf.  In  the  leaves  of  dicotyledons  (the  greater 
number  of  seed  plants)  there  is  a  large  central  vein, 
the  midrib,  running  up  the  centre  of  the  leaf,  and  this 


VASCULAR   BUNDLES  311 

usually  projects  from  the  lower  surface  (sometimes  from 
the  upper  surface  also)  and  contains  several  vascular 
bundles.  The  outer  tissue  of  the  midrib  below  the 
epidermis  commonly  consists  of  collenchyma  (p.  279). 
From  the  midrib  large  secondary  veins  branch  off,  and 
these  also  are  usually  thicker  than  the  general  thickness 
of  the  leaf,  so  that  they  project  on  the  lower  surface. 
From  the  large  secondary  veins  smaller  ones  branch, 
and  from  these  smaller  ones  still,  and  the  branches  join 
again  (anastomose),  so  that  the  whole  substance  of  the 
leaf  is  interpenetrated  by  a  network  of  vascular  bundles. 
The  smaller  veins  are  embedded  in  the  general  thickness 
of  the  leaf.  Each  of  these  smaller  bundles  is  surrounded 
by  a  sheath  of  living  cells  (bundle  sheath) ,  which  may  or 
may  not  contain  chloroplasts  (Fig.  50,  sh.).  The  finest 
branches  of  the  bundle  network  are  destitute  of  phloem 
and  each  ends  blindly  in  the  mesophyll  as  a  strand  of 
tracheids  covered  by  the  bundle  sheath  (Fig.  51,  F). 

From  these  terminal  tracheids  the  water  coming  from 
the  roots  through  the  continuous  xylem  channels  is  drawn 
by  osmosis  into  the  mesophyll  cells,  and  the  dissolved 
salts  also  diffuse,  though  independently  of  the  water 
(cf.  pp.  290, 296),  into  the  same  cells,  which  are  constantly 
using  them  up  to  form  complex  organic  substances. 
The  sugars  and  ammo-compounds,  formed  as  the  result 
of  photosynthesis,  diffuse  out  of  the  mesophyll  cells  into 
the  bundle  sheath,  and  thence  into  the  sieve  tubes,  along 
which  they  pass  into  those  of  the  larger  veins,  and  so  into 
the  midrib  and  out  of  the  leaf. 

Fibres. — The  larger  leaf  bundles  nearly  always  have 
a  strand  of  fibres,  often  crescentic  in  cross-section,  on 
the  outer  surface  of  the  phloem,  i.e.  the  surface  turned 
towards  the  lower  face  of  the  leaf,  and  there  is  sometimes 
another  similar  strand  on  the  upper  surface  of  the  xylem, 


312  THE   FOLIAGE   LEAF 

i.e.  turned  towards  the  upper  face.  In  the  larger  veins 
and  in  the  midrib  these  fibre  strands  are  frequently 
massive,  and  they  help,  sometimes  in  very  notable  degree, 
to  stiffen  the  mechanical  structure  of  the  leaf.  The  veins 
of  the  leaf  then  act  rather  like  the  ribs  of  an  umbrella. 

The  rigidity  of  the  leaf  is  partly  maintained  by  the 
turgor  of  the  living  cells  and  partly  by  the  veins,  and  these 
two  factors  vary  in  their  relative  importance  in  different 
species.1  Leaves  in  which  the  veins  are  sufficient  by 
themselves  to  keep  the  leaf  rigid  do  not  droop  even 
when  their  living  cells  are  seriously  depleted  of  water. 
In  other  cases  the  base  and  centre  of  the  leaf  is  kept  rigid 
by  the  larger  veins,  while  the  less  supported  tip  and 
edges  are  drooping.  The  edges  and  tip  also  suffer  first 
from  loss  of  water  by  evaporation  because  they  are 
farthest  from  the  source  of  supply. 

Other  Forms  of  Foliage  Leaf. — While  the  great 
majority  of  foliage  leaves  have  the  form  of  thin  plates, 
exposing  a  great  surface  in  proportion  to  their  bulk  to 
the  light  and  air,  there  are  many  leaves  which  differ 
from  this  type.  Besides  the  succulent  leaves  (see  p.  275), 
which  may  be  thick  and  flat,  or  oval  or  even  circular  in 
cross-section,  the  commonest  types  are  the  needle-shaped 
leaves  (pine)  and  the  long  bristle-shaped  leaves  of  many 
grasses,  especially  those  which  grow  in  dry  places.  The 
narrow  leaves  of  other  grasses  often  fold  on  the 
midrib,  the  stomata  being  confined  to  the  approximated 
surfaces,  or  roll  up,  in  dry  weather.  These  characters 
all  tend  to  reduce  or  to  protect  the  transpiring  surfaces, 
and  are  generally  related  to  diminished  water  supply  or 
to  diminished  water-conducting  capacity  of  the  xylem. 

1  A  thick  and  rigid  cuticle  and  a  thick-walled  hypoderm  may  also 
be  important  in  maintaining  the  rigidity  of  leaves  under  severe  water 
loss.  This  is  one  reason  why  evergreen  leaves  of  leathery  texture  such 
as  those  of  laurels  do  not  readily  droop. 


SCALE-LIKE   LEAVES  313 

Sometimes  the  leaves  are  small  and  scale-like,  though 
still  green  (cypress),  and  in  such  cases  the  stem  as  well 
as  the  leaves  may  have  a  well-developed  photosynthetic 
tissue.  Finally,  the  leaves  may  be  reduced  to  minute 
functionless  scales,  photosynthesis  and  transpiration 
being  carried  out  entirely  by  the  green  stem,  to  which 
the  photosynthetic  tissue,  resembling  in  all  respects 
the  mesophyll  of  a  typical  leaf,  is  then  confined. 

Occasionally  in  such  cases  the  leaves  are  represented 
by  bristles  or  spines  instead  of  scales. 

PRACTICAL  WORK. 

(1)  Make  a  clean  cut  under  water  across  the  base  of  the  leaf 
stalk  of  a  geranium  (Pelargonium)  leaf,  and  place  it  in  a  glass 
with  the  cut  end  dipping  below  the  surface  of  aqueous  eosin 
solution.     After  a  time  observe  that  the  eosin  has  been  sucked  up 
the  xylems  of  the  vascular  bundles  and  eventually  appears  in 
the  leaf  veins. 

(2)  Examine  in  a  drop  of  dilute  glycerine  the  transverse  section 
of  the  typical  dorsiventral  leaf  blade  (Hellebore)  provided.    Make 
a  drawing  under  the  high  power  of  (a)  the  midrib  showing  xylem, 
phloem,  fibres,  collenchyma  and  epidermis,  (b)  of  a  typical  segment 
of  the  blade  showing  the  upper  epidermis  with  cuticle,  palisade 
cells,  spongy  tissue,  with  fewer  chloroplasts  and  large  intercellular 
spaces,  and  the  lower  epidermis  with  a  stoma  in  section. 

(3)  To  another  section  add  a  drop  of  Schulze's  solution,  and 
note  the  different  staining  of  the  cuticle  and  of  the  wall  substance 
below  it. 

(4)  Strip  off  a  piece  of  the  epidermis  from  the  surface  of  the 
leaf  of  Iris,  taking  care  that  you  get  part  of  it  at  least  free  from 
adherent  mesophyll  cells.     Mount  the  strip  in  dilute  glycerine 
with  the  outer  surface  uppermost.     Find  and  draw  a  stoma  with 
adjacent  epidermal  cells  carefully  under  the  high  power.     In 
focussing  down,  the  sides  of  the  rectangular  vestibule  first  come 
into  view,   and  below  this  the  two  guard  cells  with  stomatal 
pore  between  them. 

(5)  Now  examine  a  transverse  section  of  the  same  leaf.     Find 
and  draw  very  carefully  a  stoma,  which  is  now  seen  in  sectional 
view.     Note  especially  the  cuticular  thickenings  of  the  sides  of 


314  THE   FOLIAGE   LEAF 

the  vestibule  and  of  the  walls  of  the  guard  cells,  and  that  the 
latter  bulge  into  the  adjacent  epidermal  cells.  Note  the  inter- 
cellular spaces  and  the  mesophyll  cells  below  the  stoma. 

Examine  also  in  the  same  section  and  draw  a  vascular 
bundle  in  cross-section,  distinguishing  (a)  xylem  with  vessels  and 
tracheids,  including  the  narrow  protoxylem  elements  at  the 
extremity  of  the  bundle,  (b)  phloem  with  sieve  iubes  and  companion 
cells,  (c)  fibres.  Treat  with  aniline  chloride  or  aniline  sulphate. 
The  lignified  tissues  stain  bright  yellow. 

(6)  Cut  the  end  of  the  leaf  stalk  of  the  floating  leaf  of  Limnocharis 
across  with  a  sharp  knife,  place  the  blade  in  water,  and  suck  the 
end  of  the  stalk  vigorously.     Dark  patches   appear  in   the  leaf 
blade  owing  to  water  being  sucked  in  through  the  stomata  and 
filling  the  large  intercellular  spaces   which  communicate   with 
those  of  the  leaf  stalk. 

(7)  Examine  the  shoots  of  Gorse  ( Ulex) .     Note  the  arrangement 
of   leaves,    branches   and   buds.     The   leaves   are   narrow   and 
spinous,  but  not  so  rigid  as  the  branch  spines. 

(8)  Examine   any  other  examples   of   needle-shaped,   bristle- 
like,  folded  or  rolled  leaves  that  may  be  available,  both  on  the 
plants  and  in  section. 


CHAPTER   XIX 
THE    PRIMARY    STEM 

THE  shoot  or  subaerial  part  of  the  body  of  the  higher 
plant  consists,  as  we  have  seen,  of  stem  and  leaves 
together.  The  stem  forms  the  branching  axis  of  the 
shoot  system  and  bears  the  leaves,  which  grow  out  from 
its  sides.  It  thus  (a)  supports  and  displays  the  foliage 
leaves  in  the  light  and  air,  (6)  acts  as  the  channel  of 
communication  between  root  and  leaves,  and  (c)  supports 
and  displays  the  flowers,  in  which  the  reproductive  cells 
are  formed  and  in  which  conjugation  of  the  gametes 
takes  place. 

Tropisms. — The  erect  aerial  shoot  is  negatively  geo- 
tropic,  growing  away  from  the  centre  of  the  earth,  just 
as  the  taproot  is  positively  geotropic,  growing  towards 
it.  This  can  be  seen  in  plants  kept  in  the  dark  which 
grow  straight  upwards.  If  the  plant  is  laid  on  its  side 
in  the  dark,  the  actively  elongating  region  of  the  shoot 
bends  so  as  to  bring  the  apex  into  the  vertical  position 
again.  The  shoot  is  also  positively  phototropic,  growing 
towards  the  source  of  maximum  illumination.  These 
two  tropisms  normally  act  in  the  same  direction  and 
cause  the  shoot  to  grow  straight  upwards  as  the  opposite 
ones  cause  the  root  to  grow  downwards.  But  if  illumina- 
tion is  one-sided,  the  positive  phototropism  of  the  shoot 
overcomes  its  negative  geotropism,  so  that  it  bends 
towards  the  source  of  light.  This  can  be  seen  in  the 
case  of  many  herbaceous  plants  grown  in  pots  in  windows 


3l6  THE  PRIMARY  STEM 

which  bend  so  that  their  shoot  apices  point  towards 
the  open  sky,  and  in  plants  growing  on  the  edge  of  a 
wood  which  behave  in  the  same  way.  The  leaves  tend 
to  set  themselves  at  right  angles  to  the  source  of  light 
(diaphototropism) .  As  in  the  case  of  roots,  the  bending 
occurs  by  the  cells  on  one  side  of  the  elongating  region 
growing  faster  than  those  on  the  other,  but  the  apex 
alone  perceives  the  light,  as  can  be  seen  by  covering  the 
apices  of  some  of  a  crop  of  seedlings  (e.g.  of  canary 
grass)  with  tinfoil  caps  and  illuminating  from  one  side, 
when  bending  no  longer  takes  place  in  those  which 
are  so  covered. 

Etiolation. — When  a  normally  erect  aerial  shoot  is 
grown  in  the  dark,  it  does  not  turn  green,  the  plastids 
becoming  yellowish,  the  internodes  grow  enormously 
in  length  and  the  leaves  remain  small  and  scale-like, 
while  the  tissues  are  not  properly  differentiated.  Such 
a  shoot  is  said  to  be  etiolated. 

Structure  of  the  Aerial  Stem. — The  general  structure 
of  the  stem  depends  a  good  deal  upon  the  fact  that  it 
is  essentially  a  leaf-bearing  organ.  We  distinguish 
the  nodes  or  levels  of  insertion  of  the  leaves  (which  may 
arise  from  the  stem  singly  or  in  pairs  or  circles,  called 
whorls),  from  the  internodes  or  bare  stretches  of  stem 
between  these  levels.  The  stem  terminates  in  a  bud, 
which  is  simply  the  developing  apex  of  the  shoot  bearing 
the  developing  leaves  on  its  sides,  and  usually  covered 
by  the  incurving  of  the  partly  grown  leaves,  which  arch 
over  and  serve  to  protect  the  very  delicate  apical 
meristem  from  desiccation. 

The  minute  structure  of  the  outer  layers  of  the  stem 
resembles  that  of  the  foliage  leaf  in  more  than  one 
respect.  Thus  it  is  covered  by  an  epidermis  which  has 
the  same  character  as  the  leaf  epidermis,  and  possesses 


STRUCTURE  317 

stomata  identical  with  leaf  stomata,  though  they  are 
usually  much  fewer  in  number.  Below  this  there  is  in 
the  green  aerial  stems  of  the  great  majority  of  plants 
a  living  tissue  (cortex)  containing  chlorophyll,  though 
generally  not  highly  specialised  like  the  mesophyll  of  a 
foliage  leaf.  Very  frequently,  especially  in  the  primary 
shoots  of  woody  plants,  the  outer  cortex  is  collen- 
chymatous.  In  some  few  cases,  especially  when  the 
leaves  are  reduced  and  functionless,  the  outer  cortex, 
or  portions  of  it,  may  be  differentiated  as  palisade  tissue. 
Below  the  cortex,  as  in  the  root,  the  vascular  cylinder 
occupies  the  centre  of  the  stem.  But  the  vascular 
cylinder  of  the  stem  is  very  different  in  character  from 
that  of  the  root.  In  the  first  place  it  is  generally  very 
much  wider  (the  cortex  being  correspondingly  narrower) 
in  proportion  to  the  whole  width  of  the  axis.  Secondly, 
the  vascular  tissue  is  arranged  like  that  of  the  leaf  in 
bundles,  each  consisting  of  a  strand  of  xylem  and  a 
strand  of  phloem.  Each  of  these  bundles  has  the 
xylem  towards  the  centre  of  the  stem,  and  the  phloem 
outside,  towards  the  cortex  ;  and  all  the  bundles  (in 
the  plants  called  dicotyledons)  are  arranged  side  by  side, 
separated  by  strips  of  parenchymatous  tissue,  the  rays, 
and  forming  a  hollow  cylinder  enclosing  the  usually 
parenchymatous  pith,  which  fills  up  the  centre  of  the 
stem.  Each  bundle,  like  those  of  the  larger  leaf  veins, 
nearly  always  possesses  a  strand  of  fibres  outside  the 
phloem  (Fig.  52,  A,  B,  per.}.  These  fibrous  strands  may 
be  separated  laterally  by  parenchyma,  or  they  may  be 
joined  laterally  to  form  a  complete  fibrous  cylinder. 
This  fibrous  cylinder,  or  the  fibrous  strands  together 
with  the  intervening  parenchyma,  forms  the  pericycle 
or  outermost  layer  of  the  vascular  cylinder,  and 
immediately  outside  it  comes  the  endodermis,  whose 


THE   PRIMARY   STEM 


318 

lateral  and  horizontal  walls  do  not  always  (as  in  the 
root)  possess  a  central  band  of  cutin  (see  p.  293),  though 


FIG.  52. — Structure  of  the  primary  stem.  A,  diagram  of  cross-section 
of  stem  showing  vascular  bundles  arranged  in  a  cylinder ;  X, 
xylem ;  px.,  protoxylem  ;  P,  phloem.  B,  single  bundle  in  cross- 
section  ;  cor.,  cortex  ;  end.,  endodermis  ;  per.,  peri  cycle  (fibrous 
opposite  bundle) .  C,  diagram  of  segment  of  stem  seen  obliquely 
from  above.  The  cortex  and  conjunctive  tissue  of  the  cylinder  are 
supposed  to  be  transparent,  the  epidermis  and  endodermis  opaque. 
D,  diagram  of  the  course  of  the  leaf  trace  and  stem  bundles 
at  a  node  bearing  two  opposite  leaves  in  Clematis.  The  bundles 
towards  the  observer  are  black,  those  away  from  the  observer 
shaded.  Three  bundles  enter  the  stem  from  each  leaf.  The 
laterals  (/)  at  once  join  bundles  from  the  internode  above  (two 
of  which  fork),  the  central  bundle  (c)  of  each  trace  continues  alone 
into  the  internode  below  (joining  with  others  at  a  lower  node). 

sometimes  they  do.     The  stem  endodermis  often  differs, 
however,  in  other  ways  from  the  cells  of  the  cortex 


COURSE   OF   VASCULAR   BUNDLES  319 

outside  it,  e.g.  by  possessing  specially  large  and 
conspicuous  starch  grains,  when  it  is  called  the  "  starch 
sheath." 

The  vascular  cylinder  of  the  stem  thus  differs  in  many 
ways  from  that  of  the  root,  and  this  is  related  to  its  very 
different  conditions  of  life  and  to  the  fact  that  its 
bundles  are  primarily  the  direct  downward  continuation 
of  those  of  the  leaves  (Fig.  52,  D).  They  are  identical 
in  structure  with  the  leaf  bundles,  and  their  orientation 
is  the  same,  the  outward  position  of  the  phloem  in  the 
stem  clearly  corresponding  with  its  lower  position  in 
the  leaf. 

The  stem  bundles  do  not  pursue  a  completely  in- 
dependent course.  A  certain  number  come  in  from  each 
leaf,  they  often  branch,  and  they  always  sooner  or  later 
join  on  to  neighbouring  bundles  which  have  come 
down  through  the  internode  above  from  higher  leaves 
(Fig.  52,  D).  Traced  downwards  through  the  internode 
the  bundles  fuse  with  one  another  laterally  at  various 
levels,  so  that  they  leave  room  for  those  entering  the 
cylinder  at  the  next  node  below.  As  the  base  of  the 
stem  is '  approached  more  fusions  take  place,  so  that 
comparatively  few  bundles  enter  the  hypocotyl,  and 
these  usually  fuse  with  the  cotyledon  traces,  so  that 
only  the  latter  are  directly  continuous  with  the  vascular 
cylinder  of  the  root.  In  the  hypocotyl  or  at  the  top  of 
the  primary  root  the  cylinder  narrows,  the  pith  dis- 
appears, the  pericyclic  fibres  die  out,  and  the  xylem  and 
phloem  strands  change  their  relative  positions,  so  that 
they  come  to  be  alternate,  i.e.  situated  on  different  radial 
planes.  At  the  same  time  the  cortex  changes  its  character 
and  the  shoot  epidermis  is  replaced  by  the  piliferous  layer. 

Maintenance  of  the  Erect  Position. — The  herbaceous 
stem  is  maintained  in  an  upright  position  partly  by  the 


320  THE   PRIMARY   STEM 

turgidity  of  its  living  cells.  This  is  shown  by  the 
drooping  of  many  stems,  or  of  their  young  upper  portions, 
when  they  are  wilted.  This  source  of  rigidity  is,  however, 
supplemented  in  many  plants  by  the  collenchyma  of 
the  outer  cortex,  which  is  differentiated  early,  that  is 
fairly  close  behind  the  apical  growing  point,  and  stiffens 
the  surface  layers  of  the  stem.  This  surface  stiffening 
resists  slight  bending  strains  to  which  the  upper  part  of 
the  shoot  is  subjected  by  the  wind.  Soon  the  lignifica- 
tion  of  the  primary  xylem  elements  increases  the  support 
afforded  by  the  collenchyma.  But  the  most  important 
mechanical  support  of  most  primary  herbaceous  stems 
is  contributed  by  the  fibres  of  the  pericycle.  These  are 
thickened  later,  i.e.  further  from  the  growing  point, 
than  the  collenchyma,  after  growth  in  length  has  ceased. 
Owing  to  the  relative  narrowness  of  the  cortex  compared 
with  the  width  of  the  stem,  the  fibres  of  the  pericycle, 
which  often  have  very  thickwalls,  form  a  rigid  continuous 
or  interrupted  cylinder  not  far  from  the  surface,  and 
enable  the  stem  to  support  the  weight  of  leaves  and 
branches  and  to  resist  the  bending  strains  imposed  by 
the  wind. 

Apical  Meristem. — A  longitudinal  section  through  the 
tip  of  the  shoot  shows  the  stem  tip  composed  of  meriste- 
matic  (embryonic)  cells,  and  bearing  on  its  sides  the 
first  beginnings  of  the  leaves,  which  arise  as  projections 
on  the  surface  caused  by  locally  increased  cell  division. 
As  the  young  leaves  increase  in  size  they  usually  grow 
faster  on  the  lower  than  on  the  upper  surface,  with  the 
result  that  they  curve  over  the  growing  tip  of  the  stem 
with  its  younger  and  as  yet  smaller  leaves  (Fig.  53), 
thus  serving  to  protect  these  and  the  meristem  itself, 
which  are  not  as  yet  covered  by  a  well-developed  cuticle 
from  drying  up  and  other  injury.  This  curving  of  the 


DEVELOPMENT  BEHIND  THE  APEX       32! 

partly  developed  leaves  over  the  shoot  tip  gives  the 
characteristic  "  bud  "  structure. 

The  surface  layer  of  the  meristem  is  quite  separate 
from  the  underlying  meristematic  tissue,  i.e.  it  forms  new 
cell  walls  only  in  the  direction  perpendicular  to  the 
surface,  and  thus  gives  rise  only  to  the  epidermis  of 
the  shoot.  Below  this  surface  layer  of  the  meristem  the 
cell  divisions  give  rise  to  the  young  tissues  of  the  cortex 
and  of  the  vascular  cylinder. 

Development  of  the  Shoot  immediately  behind  the 
Apex. — As  in  the  case  of  the  root,  the  products  of  division 
of  the  meristematic  cells  away  from  the  apex  increase 
in  size  by  the  development  of  vacuoles,  and  at  the  same 
time  divide  less  frequently. 

Pith. — The  cells  in  the  centre  of  the  stem,  which  will 
give  rise  to  the  pith,  are  generally  the  first  to  lose  their 
meristematic  activity.  They  often  increase  greatly  in 
size,  partly  by  their  own  growth,  and  partly  because 
they  are  passively  stretched  by  the  active  growth  of  the 
surrounding  tissues.  This  very  frequently  results  in 
the  pith  cells  becoming  the  largest  parenchymatous  cells 
in  the  stem.  Sometimes  the  energetic  growth  of  the 
surrounding  tissues,  especially  in  length,  after  the 
growth  of  the  pith  cells  has  ceased,  leads  to  the  breaking 
and  ultimate  death  of  the  pith.  This  is  the  cause  of 
the  frequent  hollowness  of  herbaceous  stems  in  which 
the  whole  of  the  pith  or  its  central  portion  has  disappeared 
in  the  adult  stem. 

Zone  of  the  Vascular  Bundles,  etc. — Just  outside  the 
pith,  in  the  zone  which  gives  rise  to  the  vascular  bundles, 
the  rays  and  the  pericycle,  very  many  of  the  cells  soon 
cease  to  divide  by  horizontal  walls,  divisions  continuing 
only  parallel  to  the  axis  of  the  stem.  Growth  in  length 
of  these  cells  continues,  so  that  they  become  elongated. 
21 


322  THE   PRIMARY   STEM 

The  strands  of  such  elongated  cells  which  will  eventually 
form  the  vascular  bundles  are  known  as  the  desmogen 
strands  (Fig.  53,  desm.},  and  these  soon  become  distinct 
from  the  surrounding  tissue.  The  outlines  of  the  tissues 
which  will  be  formed  later  from  these  often  become  clear 
at  an  early  stage  of  development,  and  the  large  cells 
which  will  become  the  vessels  of  the  xylem  become 
apparent.  About  the  same  time,  or  even  earlier, 
the  limit  between  the  vascular  cylinder  and  the  cortex 
becomes  evident,  owing  to  differentiation  between  peri- 
cycle  and  cortex,  the  cells  of  the  latter  -continuing  to 
grow  in  width. 

Development  of  Leaves  and  Axillary  Buds.— While 
these  changes  have  been  taking  place  in  the  stem  tissue, 
the  leaves,  which  first  appear  very  close  to  the  stem 
apex  as  small  papillae,  or,  where  the  adult  leaves  have 
a  broad  insertion  on  the  stem,  as  curved  ridges  of 
meristematic  tissue,  have  been  growing  in  length  and 
breadth.  In  the  axil  of  each  leaf  there  arises,  sooner  or 
later,  another  papilla  of  meristematic  cells,  the  rudiment 
of  the  axillary  bud  (Fig.  53)  ;  and  upon  the  sides  of  this 
there  arise  the  rudiments  of  the  first  leaves  of  the  lateral 
shoot  into  which  the  bud  may  develop.  The  bud  may 
grow  out  at  once  to  form  a  branch,  not  much  behind  the 
main  shoot  in  development.  On  the  other  hand,  it  may 
remain  a  bud  for  a  long  time,  even  for  many  years  ;  and 
in  some  cases  it  never  develops  further  at  all.  Its  fate 
depends  on  various  external  and  internal  conditions. 

Differentiation  of  the  Tissues. — The  first  tissue  to 
attain  the  adult  form  is  the  protoxylem  of  the  vascular 
bundles.  Close  to  the  inner  limit  of  the  desmogen  strand 
certain  elongated  cells  become  spirally  thickened  (see 
p.  280),  and  the  thickening  band  rapidly  becomes  lignified, 
the  protoplasm  then  dying.  This  happens  at  first  at 


DEVELOPMENT  BEHIND  THE  APEX 


323 


or  just  below  the  node  of  a  leaf,  in  the  largest  central 
bundle  of  the  leaf  trace,  and  the  differentiation  of  the 
protoxylem  progresses  in  both  directions,  downwards 


FIG.  53. — Diagrams  of  the  differentiation  of  a  stem  behind  the  apex. 
On  the  left  is  a  diagram  of  a  longitudinal  section,  on  the  right 
of  transverse  sections  at  the  levels  A,  B  and  C  :  /,  leaf  ;  ax. bud., 
axillary  buds ;  desm.,  desmogen  strand ;  px.,  protoxylem.  For 
further  explanation  see  text. 

into  the  internode  below  and  upwards  into  the  leaf 
(Fig.  53,  px.).  In  other  words,  the  elongated  cells  in  a  line 
with  the  first  formed  spiral  tracheids  likewise  become 
converted  into  spiral  tracheids,  so  that  a  strand  of  proto- 


324  THE   PRIMARY   STEM 

xylem  is  formed  which,  so  to  speak,  grows  in  length  at 
each  end,  extending  up  into  the  leaf  and  down  into  the 
stem,  and  these  strands  form  the  first  water  channels 
of  the  shoot.  Later,  protoxylem  is  formed  in  the  smaller 
bundles,  and  the  strands  extend  till  they  meet  those  of 
other  bundles,  and  thus  form  a  continuous  water- 
conducting  system. 

Elongating  Region  of  the  Stem.— The  formation  of  the 
protoxylem  takes  place  at  the  beginning  of  the  elongating 
region  of  the  stem,  which  is  much  longer  than  that  of 
the  root,  and  often  extends  over  several  centimetres  or 
even  several  inches.  It  is  the  internodes  which  elongate, 
separating  the  successive  leaves  from  one  another,  the 
nodal  regions  growing  mainly  in  diameter. 

Meanwhile  the  other  tissues  of  the  shoot  are  rapidly 
differentiating.  The  cuticle  as  it  is  exposed  to  the  air 
by  elongation  of  the  internodes  becomes  thicker  ;  the 
outer  layers  of  the  cortex  frequently  become  collenchy- 
matous  by  the  thickening  of  the  longitudinal  walls  at 
the  corners  of  the  cells  ;  chlorophyll  develops  in  the 
plastids  ;  and  intercellular  spaces  between  many  of  the 
cortical  cells. 

In  the  vascular  cylinder  the  fibres  of  the  pericycle 
are  clearly  marked  at  an  early  stage,  but  their  walls 
are  not  thickened  till  relatively  late,  generally  after 
elongation  has  ceased.  f  Narrow  sieve  tubes  (protophloem) 
usually  appear  early  just  below  the  pericyclic  fibres  and 
opposite  the  protoxylem  of  each  bundle.  When  growth 
in  length  has  ceased,  the  development  of  the  primary 
vascular  bundles  continues  by  the  centripetal  forma- 
tion of  the  larger  sieve  tubes  and  companion  cells 
(metaphloem) ,  and  of  metaxylem,  consisting  of  larger 
scalariform  or  some  other  type  of  pitted  vessels.  Differ- 
entiation progresses  outwards  (centrifugally)  in  the 


MODIFIED    FORMS   OF   SHOOT  325 

primary  xylem,  the  pitted  vessels  following  on  the 
spiral  tracheids  of  the  protoxylem.  A  band  of  tissue 
between  the  xylem  and  phloem  remains  undifferentiated, 
and  this  is  the  seat  of  the  cambium  or  secondary  vascular 
meristem  in  stems  which  undergo  secondary  thickening. 

The  tissues  of  the  leaf  keep  pace  in  differentiation 
with  those  of  the  stem,  the  leaf  blade  expanding,  while 
the  epidermis,  veins  and  mesophyll  acquire  their  adult 
characters. 

Modified  Forms  of  Shoot. — While  the  typical  aerial 
shoot  stands  erect  in  the  air,  other  forms  do  not  develop 
the  mechanical  tissues  required  to  maintain  the  erect 
position.  Some  of  these  trail  on  the  ground,  others 
twine  round  any  support  (such  as  the  erect  stem  of  an 
upright  plant,  the  side  of  the  stem  away  from  the  support 
growing  faster  than  the  side  touching  it) .  Other  shoots, 
as  we  have  already  seen  (Chapter  XV),  grow  below  the 
surface  of  the  soil  and  do  not  bear  foliage  leaves,  but 
only  scale  leaves.  Others,  again,  grow  very  little  in 
length  but  increase  in  diameter  ("  rootstocks,"  corms, 
tubers).  All  these  types  of  shoot  have  the  same  essential 
plan  of  construction  as  the  typical  erect  aerial  stem, 
but  they  differ  very  widely  in  details.  Thus  the  cortex 
may  be  relatively  very  broad,  and  it  may  contain 
vascular  bundles  in  addition  to  those  of  the  cylinder 
In  some  cases  the  vascular  cylinder  early  loses  its 
identity  altogether,  so  that  the  bundles  are  scattered 
through  the  stem.  Chlorophyll  is  not  developed  in 
subterranean  shoots,  nor  do  they,  as  a  rule,  bear  stomata. 
Adventitious  roots  are  practically  always  produced  on 
subterranean  shoots,  and  very  often  also  on  creeping 
shoots,  especially  at  the  nodes,  as  well  as  from  the 
bases  of  erect  aerial  shoots  below  the  surface  of 
the  soil. 


326  THE   PRIMARY   STEM 

PRACTICAL  WORK 

(1)  Sketch  the  piece  of  herbaceous  shoot  provided,  marking 
leaf  blade,  leaf  stalk,  axillary  buds,  nodes,   internodes.     Note  and 
indicate  the  arrangement  of   the   vascular   bundles  as   seen   on 
the  cut  surfaces  of  the  stem  and  leaf  stalk  :  also  the  epidermis, 
cortex,  pericycle,  rays  and  pith.     [A  piece  of   the  mature  shoot 
of  almost  any  large  herbaceous  plant  is  suitable,  e.g.  sunflower, 
vegetable  marrow,  bean.]     Examine  also  the  preparation  of  a 
piece  of  shoot  made  transparent  in  Canada  balsam  so  that  the 
course  of  the  vascular  bundles  can  be  followed. 

(2)  Pull  off  the  outer  leaves  of  the  Brussels  sprout  provided. 
This  is  a  bud  whose  outer  leaves  become  mature  with  very  little 
elongation  of  internodes.     With  a  sharp  knife  or  a  razor  cut  a 
longitudinal  section  as  accurately  as  possible  through  the  tip,  and 
make  a  diagrammatic  sketch  showing  (a)  growing  point  (primary 
meristem),  (&)  the  youngest  leaves,  (c)  older  leaves  with  buds  in 
their  axils,  (d)  young  vascular  bundles.     Pull  off  the  remaining 
leaves  and  note  their  insertions  and  their  axillary  buds. 

(3)  Examine  the  prepared  slide  of  the  same   for   microscopic 
details.     Note    especially    the    origin    of    the    vascular    bundles 
(desmogen  strands),  the  first  spiral  tracheids  of  the  protoxylem 
and  the  general  appearance  of  the  pith  cells. 

(4)  Examine  in  dilute  glycerine  a  cross-section  of  the  stem  of 
the  sunflower,  at  first  with  the  naked  eye  and  with  a  hand  lens ; 
and  then  draw  a  diagram  with  the  help  of  the  low  power.     Mark 
the  outlines  of  the  following  tissues  and  tissue-systems  :  epidermis, 
outer  cortex,  inner  cortex,  endodermis  (starch  sheath)  ;   vascular 
cylinder,  consisting  of  pericycle  (composed  of  fibres  opposite  the 
bundles,    parenchymatous    between),    rays,    pith,    and    vascular 
bundles,  showing  xylem,  phloem  and  beginning  of  cambium. 

Put  a  section  in  a  drop  of  Schulze's  solution  for  a  few  minutes, 
and  note  the  colour  reactions  of  the  various  tissues,  showing  the 
distinction  between  cellulose  and  lignified  tissues. 

(5)  Now  make  careful  high-power  drawings  of  a  few  cells  of 
each  of  the  following  tissues  :  (a)  epidermis  with  cuticle  ;  (b)  outer 
cortex    (collenchymatous)  ;     (c)    inner    cortex  (parenchymatous)  ; 
(d)  endodermis  ;    (e)  pericycle  fibres  ;    (/)  phloem,  including  sieve 
tubes,  companion  cells  and  parenchyma;   (g)  cambium  ;   (h)  xvlem, 
including   large  (pitted)  vessels,   parenchyma   and   xylem  fibres ; 
and   (i)  protoxylem  ;   (j)   cells  of   the   medullary  rays  which  are 
beginning  to  form  interfascicular  cambium. 

Examine  a  prepared  stained  section  of  the  same  to  verify 
details  and  compare  the  staining. 


PRACTICAL   WORK  327 

(6)  Examine  the  radial  longitudinal   section,  and  draw  under 
the  high  power  a  sample  of  the  following  tissues  :   (a)  phloem, 
showing    sieve    tube    (with  sieve   plate)    and    companion    cell ; 
(b)  metaxylem,  showing  part  of  a  pitted  vessel  with  adjoining 
parenchyma  and  fibres  ;    (c)  protoxylem,  showing  spiral  vessels 
and   adjoining  thin-walled  parenchyma.     [It  is  necessary  that 
the  section  should  pass  radially  through  a  bundle  in  order  that 
the  above  tissues  should  appear  in  it.] 

(7)  Examine   the   demonstration   showing   positive   phototro- 
pism  and  the  fact  that  the  apex  of  the  shoot  is  the  light-perceiving 
organ   [capped  Setaria  seedlings  are  suitable]  :     also    examples 
of  etiolated  shoots  to  compare  with  examples  grown  in  light 
[bean  plants  are  suitable]. 


CHAPTER   XX 

THE  WOODY  STEM 

General    Characters  of   Woody   Plants.— We    saw   in 

Chapter  XV  that  perennial  herbaceous  plants  maintain 
their  vegetative  bodies  from  year  to  year  by  means  of  a 
persistent  underground  shoot  (rhizome,  tuber,  corm  or 
bulb) ,  which  throws  up  new  aerial  shoots  at  the  beginning 
of  the  next  growing  season.  New  portions  of  the  rhizome 
are  formed  every  year,  new  tubers,  corms  or  bulbs  are 
produced,  the  older  parts  of  the  rhizome,  the  old  tubers, 
corms  and  bulbs  dying  off.  The  aerial  shoots  of  woody 
plants,  on  the  other  hand  (trees  and  shrubs),  do  not  die 
down  every  year,  leaving  only  the  rhizome  or  other 
form  of  underground  shoot  to  continue  growth  the 
next  year,  but  themselves  continue  growing  from  year 
to  year,  the  terminal  (or  lateral)  buds  of  the  branches 
forming  a  fresh  portion  of  shoot  in  each  growing  season, 
this  being  a  direct  (or  indirect)  continuation  of  the  portion 
formed  the  year  before.  At  the  same  time  the  portions 
of  stem  formed  in  earlier  years  grow  in  thickness  each 
year,  and  are  thus  able  both  to  support  and  to  conduct 
water  and  salts  from  the  root  to  the  constantly  increasing 
shoot  system  above.  This  process  of  growth  in  thickness 
is  called  secondary  thickening,  and  is  brought  about  by 
the  activity  of  the  cambium  or  secondary  (vascular) 
meristem,  whose  beginnings  we  have  already  noted. 
In  most  climates,  also,  there  is  a  definite  season  of  the 
year  unfavourable  or  impossible  for  growth  (either  a 


WINTER   BUDS  329 

cold  winter  or  a  dry  season),  and  the  shoots  of  a  woody 
perennial  must  be  protected  from  drying  up  during 
that  period,  and  this  is  effected  by  another  secondaiy 
meristem,  the  cork-cambium. 

The  main  structural  features  in  which  a  woody 
perennial  differs  from  a  herbaceous  plant  are  therefore 
three.  First,  it  produces  secondary  vascular  and 
supporting  tissue.  Secondly,  it  has  means  of  protection 
(bark)  of  its  general  shoot  surface  during  the  unfavour- 
able season.  Thirdly,  its  aerial  buds  (winter  buds) 
are  protected  during  the  same  period. 

Winter  Buds. — At  the  tips  and  on  the  sides  of  the 
younger  portions  of  the  branches  of  a  tree  or  shrub 
buds  are  to  be  seen  during  the  winter  covered  with 
brown  scales.  These  bud  scales  are  modified  leaves 
which  were  formed  at  the  end  of  the  last  growing  season, 
and  they  completely  cover  in  the  delicate  tissue  (apical 
meristem)  in  the  interior  of  the  bud.  The  winter  bud 
scales  are  largely  composed  of  cells  with  corky  (water- 
proof) walls,  and  other  (gland)  cells  are  often  present 
which  secrete  a  resin  or  gum  that  glues  the  scales 
together  and  thus  renders  the  bud  additionally  water- 
tight. This  arrangement  not  only  prevents  the  tissues 
within  from  drying  up,  but  also  stops  rain  from  soaking 
in  and  rotting  the  tissues. 

Within  the  bud  are  young,  partly  developed  foliage 
leaves,  and  sometimes,  in  addition,  a  group  of  ready 
formed  flower  buds  (inflorescence).1  In  the  case  of 
fruit  trees,  such  as  the  apple  and  pear,  those  winter  buds 
which  contain  the  young  flowers  are  called  fruit  buds, 
because,  of  course,  it  is  the  potential  production  of  fruit 
from  the  flowers  which  interests  the  fruit  gardener. 

1  Compare  the  buds  on  the  crocus  corm  and  in  the  tulip  bulb,  which 
are  also  winter  buds,  though  borne  on  an  underground  shoot. 


330  THE   WOODY  STEM 

They  can  be  distinguished  from  the  "  leaf  buds  "  (buds 
of  shoots  which  will  produce  leaves,  but  no  flowers)  at 
any  time  during  the  winter  in  such  fruit  trees  as  the 
pear,  and  very  easily  in  early  spring  when  they  have 
begun  to  swell,  by  their  more  rounded  form. 

In  the  spring  the  tissue  of  the  axis  begins  to  grow, 
and  this  growth  eventually  bursts  the  bud  and  pushes 
out  the  foliage  leaves,  which  unfold  and  grow  to  their 
full  size,  while  the  internodes  of  the  stem  elongate. 
If  there  are  young  flowers  in  the  bud,  these  also  grow 
and  open.  The  winter  bud  scales  fall  off,  leaving  a 
zone  of  scars  on  the  stem,  and  these  will  clearly  mark 
the  junction  of  last  year's  stem  and  the  stem  formed 
during  the  present  growing  season.  At  the  close  of 
the  growing  season  the  foliage  leaves  fall  off,  and  the 
terminal  bud  (or  sometimes  a  lateral  which  takes  the 
place  of  the  terminal)  once  more  passes  into  the  winter 
condition  by  forming  winter  bud  scales  which  close  it 
in.  These  again  will  fall  off  and  leave  another  zone  of 
scars  in  the  next  spring.  In  this  way  successive  zones  of 
bud  scale  scars  are  left  on  the  stem,  and  the  interval 
between  two  of  these  zones  represents  a  year's  growth 
of  the  branch.  By  noting  the  positions  of  these  zones 
the  history  of  a  woody  branch  can  be  read  for  some  years 
back.  Eventually  the  scars  are  destroyed  by  the  scaling 
off  of  the  bark. 

Some  of  the  axillary  buds  formed  on  the  current 
year's  shoot  may  grow  out  during  the  growing  season 
to  form  branches.  The  rest  form  winter  buds,  and  of 
these  some  will  grow  out  next  spring,  but  others 
remain  small  (dormant  buds] ,  and  will  only  grow  out  later 
on,  under  some  special  stimulus,  such  as  may  result,  for 
instance,  from  the  destruction  of  the  terminal  shoot. 

Secondary    Meristems.— Growth    in    thickness    of    a 


SECONDARY   MERISTEMS  331 

woody  stem  is  carried  on  by  the  activity  of  a  secondary 
(vascular)  meristem  (cambium)  which  arises  in  the  un- 
differentiated  tissue  between  the  xylem  and  the  phloem 
of  each  primary  bundle,  and  extends  across  the  rays 
to  form  a  continuous  ring  x  of  meristematic  (actively 
dividing)  tissue  which  runs  round  the  stem.  By  far  the 
greatest  part  of  the  mass  of  a  tree-trunk  is  formed  of 
secondary  xylem,  which  is  the  wood  of  the  trunk,  by  the 
activity  of  the  cambium. 

Another  secondary  meristem,  the  cork  cambium  or 
phellogen2,  arises  in  the  stems  and  roots  of  most  woody 
plants,  external  to  the  vascular  tissues.  In  stems  it 
arises  most  often  in  the  outermost  layer  of  the  cortex, 
immediately  below  the  epidermis,  but  sometimes  in  the 
epidermis  itself,  in  the  pericycle,  or  in  a  deep  layer  of  the 
cortex.  This  forms  the  tissue  (cork)  of  which  the  outer 
bark  of  a  tree  or  shrub  is  largely  or  wholly  composed. 

The  Cambium  and  Secondary  Thickening. — The  cam- 
bium consists  of  active  meristematic  cells,  typically 
elongated  in  the  direction  of  the  axis  of  the  stem,  and 
also  tangentially,  while  they  are  often  narrow  in  the 
radial  direction.  The  top  and  bottom  walls  of  each 
cell  are  commonly  inclined  like  a  roof  or  penthouse, 
the  slopes  being  directed  tangentially  (Fig.  54,  A). 
Properly  speaking,  the  term  cambium  is  confined  to  a 
single  layer  of  cells,  which  continually  divide  by  new 
tangential  walls  (Fig.  54,  A-C),  one  of  the  daughter  cells 
at  each  division  becoming  a  tissue  mother  cell,  giving 
rise  to  vascular,  fibrous  or  parenchymatous  tissue 
elements  of  the  secondary  xylem  or  phloem,  according 
to  the  side  on  which  it  is  cut  off,  the  other  growing  again 
to  form  the  cambial  cell.  The  term  is  often,  however, 

1  As  seen  in  cross-section  :    it  is  really,  of  course,  a  cylinder. 
*  Greek  ^eAAog,  cork,  and  yevvdw,  produce. 


332  THE   WOODY   STEM 

loosely  applied  not  only  to  the  cambial  (initial)  layer 
itself,  but  also  to  the  undifferentiated  tissue  mother 
cells  on  each  side  of  it.  When  cell  division  is  rapid 
relatively  to  the  rate  of  differentiation,  there  are  several 
layers  of  this  undifferentiated  tissue  between  the  fully 
formed  secondary  xylem  and  phloem  (Fig.  54,  E). 

The  cambial  layer  in  the  bundle  itself  (interfascicular 
cambium)  is  really  a  layer  of  the  desmogen  strand  which 
has  remained  meristematic,  and  the  process  of  cell 
division  and  differentiation  of  the  products  (tissue 
mother  cells)  into  secondary  xylem  and  phloem  elements 
is  a  direct  continuation  of  the  cell  division  and  differentia- 
tion, which  has  already  taken  place  in  the  desmogen 
strand,  producing  primary  xylem  and  phloem,  and 
proceeding  outwards  from  the  protoxylem  and  inwards 
from  the  protophloem.  In  the  case  of  the  xylem 
especially  the  process  often  goes  straight  on  with  no 
pause,  the  elements  of  the  secondary  xylem  continuing 
in  the  same  radial  rows  as  those  of  the  primary  xylem, 
so  that  it  is  impossible  to  say  where  the  one  ends  and  the 
other  begins.  In  some  cases,  however,  there  is  a  distinct 
pause  between  the  completion  of  the  primary  and  the 
beginning  of  secondary  development. 

Each  tissue  mother  cell  cut  off  from  a  cambial  cell 
on  the  inside,  i.e.  towards  the  primary  xylem,  may 
become  either  (a)  a  tracheid,  (b)  a  segment  of  a  vessel, 
(c)  a  xylem  fibre,  or  (d)  by  horizontal  division  a  vertical 
file  of  parenchyma  cells  (Fig.  54,  D).  The  cells  undergo 
the  corresponding  modifications  of  the  cell  wall,  the 
protoplasm  dying  in  all  except  the  last  case.  Each 
tissue  mother  cell  cut  off  from  a  cambial  cell  on  the 
outside,  i.e.  towards  the  primary  phloem,  may  become 
either  (a)  a  segment  of  a  sieve  tube  with  one  or  more 
companion  cells,  (b}  a  phloem  fibre,  or  (c)  by  horizontal 


ACTIVITY   OF   THE   CAMBIUM 


333 


FIG.  54. — Activity  of  the  cambium.  A-D,  diagrams  of  cambial 
cell  and  its  divisions  (inner  and  outer — tangential — walls  supposed 
to  be  transparent,  lateral — radial — walls  opaque).  A,  single 
cambial  cell  seen  obliquely  from  within.  B,  first  tangential 
division  (i).  C,  second  tangential  division  (2);  c,  cambial  cell; 
x,  xylem  tissue  mother  cell ;  p,  phloem  tissue  mother  cell.  D, 
diagram  of  tissue  mother  cell  which  has  divided  horizontally 
to  form  a  vertical  file  of  short  parenchyma  cells.  E,  part  of 
cambium  and  tissues  it  has  produced  as  seen  in  transverse  section 
under  high  power  ;  c,  cambial  cells ;  v,  xylem  vessels ;  t,  tracheid ; 
m.r.,  medullary  ray  cells ;  s.t.,  young  sieve  tube ;  p.p-,  phloem 
parenchyma  ;  w.p.,  wood  parenchyma ;  /,  fibres. 


334  THE   WOODY  STEM 

division  a  vertical  file  of  phloem  parenchyma  cells. 
The  xylem  fibres  are  usually  more  strongly  lignified 
than  the  phloem  fibres,  and  the  xylem  parenchyma  is 
often,  though  not  always,  thick  walled  and  lignified. 
The  tracheids  and  fibres  commonly  elongate  very  much 
in  developing  from  the  tissue  mother  cells,  and  their 
pointed  ends  slide  past  one  another,  so  as  to  overlap 
very  considerably. 

Secondary  Rays. — Certain  cambial  cells,  instead  of 
forming  any  of  the  above-mentioned  elements,  cut  off 
cells  on  both  sides,  which  then  divide  horizontally, 
each  cell  of  the  file  so  formed  sometimes  elongating 
radially,  so  that  it  corresponds  on  each  side  with 
several  elements  of  the  secondary  xylem  or  phloem. 
Several  cambial  cells  adjacent  in  the  vertical  direction 
(sometimes  also  several  adjacent  in  the  tangential 
direction)  behave  simultaneously  in  this  way,  and  once 
a  set  of  cambial  cells  has  begun  to  form  them,  it  does 
not  produce  anything  but  this  kind  of  cell.  In 
this  way  vertical  plates  of  tissue  called  secondary  rays 
are  formed,  running  through  the  secondary  vascular 
tissue  in  a  radial  direction,  several  or  many  cells  deep 
(i.e.  in  the  vertical  direction)  and  one  or  several  cells 
broad  (i.e.  in  the  tangential  direction) .  As  the  secondary 
xylem  increases  in  bulk  it  also  of  course  increases  in 
circumference,  and  the  cambium  increases  in  circum- 
ference with  it.  This  it  does  by  the  radial  division  of 
its  cells,  i.e.  by  putting  new  cells  into  its  circumference. 
Every  year  fresh  secondary  rays  are  begun,  while  those 
already  begun  are  continued,  so  that  in  proceeding  from 
the  centre  to  the  circumference  of  the  secondary  xylem 
of  a  woody  stem  several  years  old,  the  number  of  rays 
cut  by  successively  larger  circles  continually  increases, 
and  the  secondary  tissue,  as  seen  in  cross-section,  is 


SPRING    WOOD    AND   AUTUMN    WOOD  335 

divided  up  into  radial  wedges  by  the  rays.  The  rays 
are  generally  joined  by  tangential  bands  of  xylem 
parenchyma,  so  that  the  wood  is  divided  into  relatively 
small  blocks  of  tracheids  and  vessels,  each  in  contact 
with  living  parenchyma  cells.  It  is  probable  that  this 
fact  is  related  to  the  mechanism  of  conduction  of  water 
and  other  substances  through  the  wood,  which  is  not 
fully  understood.  Accompanying  the  rays  are  hori- 
zontally running  intercellular  channels  which  com- 
municate with  the  cortical  system  of  intercellular  spaces, 
and  thus  serve  to  aerate  the  living  cells  of  the  secondary 
xylem.  Broad  rays  (principal  rays)  are  often  formed 
as  continuations  of  the  primary  rays  (separating  the 
primary  bundles)  when  these  are  narrow,  and  can  be 
distinguished  from  the  narrow  subordinate  rays  formed 
opposite  the  primary  bundles  (Fig.  55,  C). 

Annual  Rings. — The  well-known  concentric  rings  seen 
on  the  cut  stump  of  a  felled  tree  are  the  expression  of  the 
fact  that  the  cambium  does  not  form  perfectly  uniform 
wood  throughout  the  year.  In  the  early  summer,  when 
its  activity  begins,  it  forms  numerous  and  often  large 
vessels  with  accompanying  parenchyma  ("  spring  wood"). 
These  provide  new  conducting  channels  for  taking  water 
and  salts  up  to  the  new  foliage.  In  the  late  summer, 
after  these  channels  have  been  provided  and  the  foliage 
is  no  longer  increasing,  the  vessels  are  smaller  and  fewer 
in  number,  and  the  bulk  of  the  wood  produced  is  often 
composed  of  fibres,  the  water-conducting  elements  being 
sometimes  quite  absent  ("  autumn  wood  ").  The  contrast 
between  the  alternating  zones  of  spring  and  autumn 
wood  gives  the  rings  which  can  be  counted  with  the'naked 
eye  on  the  surface  of  a  tree-stump.  A  check  to  growth, 
for  instance  a  severe  drought  at  midsummer  or  the 
wholesale  destruction  of  the  foliage  by  caterpillars, 


336 


THE   WOODY   STEM 


De.con.dary  xylem 


Primary  xyle 


FIG.  55. — Diagrams  of  secondary  thickening  in  the  stem.  A,  cross- 
section  of  stem  with  bundles  separated  by  broad  primary  rays 
in  the  first  year  of  thickening  ;  secondary  xylem  and  phloem 
are  formed  all  round  the  cylinder,  bridging  the  broad  primary 
rays.  B,  ditto,  with  bundles  separated  by  narrow  primary  rays  ; 
the  cambium  continues  the  primary  rays  as  secondary  (principal) 
rays;  s.t.,  secondary  tissue.  C,  section  of  stem  after  six  years 
of  thickening ;  the  primary  rays  are  continued  as  principal  rays, 
and  new  narrower  subordinate  rays  are  formed  in  the  wedges  of 
secondary  tissue  formed  opposite  the  primary  bundles. 


SAPWOOD   AND   HEARTWOOD  337 

followed  by  the  outgrowth  of  buds  and  the  unfolding  of 
fresh  leaves,  will  give  rise  to  a  double  ring  in  one  year, 
so  that  the  estimation  of  the  age  of  a  tree  by  counting 
the  rings  is  not  always  quite  accurate.  Sometimes  a 
similar  annual  alternation  of  bands  of  sieve  tubes  (spring) 
and  fibres  (autumn)  can  be  detected  under  the  micro- 
cope  in  the  secondary  phloem. 

Sapwood  and  Heartwood. — When  a  tree-trunk  has 
reached  a  certain  age,  which  varies  in  different  species 
of  tree,  the  wood  nearest  the  centre  undergoes  certain 
changes,  which  gradually  spread  outwards  as  increase 
in  thickness  continues.  The  living  cells  (rays  and 
xylem  parenchyma)  die  and  the  walls  of  the  lignified 
cells  lose  water,  increase  in  hardness  and  usually  change 
in  colour,  often  becoming  darker  (oak),  and  sometimes 
producing  a  pigment  which  stains  them  a  distinct  colour 
(e.g.  black  in  ebony,  yellow  in  satin  wood  and  yellow- 
wood,  purple  in  logwood)  ;  this  harder  internal  wood 
is  called  heartwood.  It  no  longer  functions  as  a  water- 
conducting  tissue,  and  it  is  the  wood  most  suitable  for 
use  in  building,  furniture  making,  etc.,  because  it  does 
not  warp  by  losing  much  water  through  evaporation  on 
drying.  The  wood  nearest  the  cambium  which  is  still 
conducting  water  and  still  contains  living  cells  is  called 
sapwood.  The  sapwood  forms  a  belt  of  approximately 
constant  width,  continually  added  to  on  the  outside  by 
the  cambium,  and  continually  converted  into  heartwood 
on  the  inside. 

Some  trees  have  no  heartwood,  the  dead  wood  in  the 
centre  remaining  soft  and  often  decaying  after  it  dies,  as 
in  the  willow.  This  is  frequently  the  cause  of  hollow 
trunks,  though  hard  heartwood  too  sometimes  decays. 

Cork  Formation. — The  phellogen  or  cork  cambium 
arises  in  most  cases  from  the  outermost  layer  of  cortical 

22 


33§  THE   WOODY   STEM 

cells  (Fig.  56,  a),  i.e.  immediately  below  the  epidermis, 
sometimes  from  the  epidermis  itself,  occasionally  in 
a  deeper  layer  of  the  cortex,  and  fairly  often  in  the 
pericycle.  In  roots  the  phellogen  is  nearly  always 
formed  in  the  pericycle. 

Each  phellogen  cell  arises  as  the  result  of  two  rapidly 
following  divisions  of  the  living  tissue  cell  in  which  it 
is  formed,  the  two  new  parallel  walls  being  tangentially 


cork 


OCD 


FIG.  56. — Cork  formation :  a,  b,  Beginning  of  phellogen  in  the  outer- 
most layer  of  cortex  ;  ep.,  epidermis  ;  phel.,  phellogen.  c,  Seven 
layers  of  cork  formed  by  phellogen  below  epidermis,  d,  Diagram 
of  two  cork  cells  showing  the  three  layers  of  the  wall  (see  text). 

directed  and  cutting  out  the  phellogen  cell  between  them 
(Fig.  56,  b,phel.).  The  phellogen  cell  then  continues  to 
divide,cutting  off  a  series  of  daughter  cells  on  the  outside, 
i.e.  towards  the  surface  of  the  stem,  forming  a  regular 
radial  row,  and  the  walls  of  these  are  thickened,  and  be- 
come partially  converted  into  corky  substance  (Fig.  56,  c). 
The  middle  lamellae  remain  unchanged,  the  next  layers 


CORK   AND   LENTICELS  339 

become  corky  (a  complex  substance,  suberin,  of  which 
fatty  acids  are  constituents,  being  deposited  in  them), 
and  a  cellulose  layer  is  laid  down  next  the  cell  cavity 
(Fig.  56,  d).  The  cork  cell  then  dies,  for  the  corky  cell 
wall  is  waterproof  and  prevents  the  diffusion  of  water 
and  solutes  into  and  out  of  the  cell  necessary  to  the 
continued  life  of  the  protoplasm.  Brown  colouring 
substances  are  usually  formed  both  in  the  walls  and  in 
the  cavities  of  the  cork  cells.  The  thick  layer  of  cork 
cells  with  waterproof  walls  forms  a  very  effective  barrier 
between  the  living  tissues  of  the  plant  and  the  outer 
world.  It  is  effective  not  only  in  checking  evaporation 
and  gaseous  interchange  in  general,  but  also  in  stopping 
the  attacks  of  minute  insects  and  the  germ  tubes  of 
fungal  spores.  Like  the  epidermis,  however,  which  it 
supersedes,  it  is  interrupted  at  certain  spots  by  openings, 
and  these  are  called  lenticels. 

Lenticels  are  formed  in  the  following  way  :  Cork 
formation  typically  begins  below  the  stomata  at  a 
somewhat  deeper  level  of  the  cortex  than  over  the  rest 
of  the  surface  of  the  stem,  and  thence  it  spreads  to  the 
outermost  cortical  layer  (or  to  the  epidermis  itself)  over 
the  general  surface.  The  vigorous  cell  division  below 
the  stomata  raises  the  epidermis  at  these  points  and 
eventually  bursts  it.  The  cork  cells  formed  below  the 
stoma  are  loose,  with  abundant  intercellular  spaces 
between  them,  and  this  loose  brown  tissue  is  pushed  out 
through  the  opening  made  by  the  bursting  of  the  stoma, 
and  forms  a  purulent  projection  on  the  surface.  The 
whole  structure  is  called  a  lenticel,  and  through  the 
air  spaces  between  the  cells  of  the  lenticellar  cork, 
which  communicate  with  the  intercellular  spaces  of  the 
cortex  below,  diffusion  of  gases  takes  place  between  the 
living  cells  of  the  stem  and  the  outside  air. 


340  THE   WOODY   STEM 

Bark. — The  original  phellogen  does  not,  as  a  rule, 
remain  active  indefinitely.  In  most  cases  it  stops 
dividing  after  some  years,  and  a  new  phellogen  is  formed 
in  a  deeper  layer  of  the  cortex,  producing  a  second  layer 
of  cork,  separated  from  the  first  by  some  layers  of  cortical 
cells.  This  second  layer  is  succeeded  by  a  third,  after 
another  interval,  at  a  still  deeper  level,  and  so  on  till 
the  new  phellogen  come  to  be  formed  in  the  parenchyma 
cells  of  the  secondary  phloem.  The  later  formed 
phellogens  cut  deeper  and  deeper  into  the  secondary 
phloem,  the  older  parts  of  which,  together  with  the 
pericycle  and  cortex  (the  whole  mass  being  called  the 
outer  bark),  are  thus  killed,  being  cut  off  from  the  living 
and  growing  tissues  of  the  stem  on  each  side  of  the 
cambium.  The  zone  of  functional  living  phloem  (inner 
bark)  thus  remains  approximately  constant  in  width, 
being  continually  reduced  on  the  outside  by  the  forma- 
tion of  deeper  and  deeper  layers  of  cork,  and  continually 
added  to  on  the  inside  by  the  cambium.  This  is  a 
parallel  phenomenon  to  the  formation  of  heartwood 
from  the  older  secondary  xylem.  In  an  old  tree-trunk 
only  a  comparatively  narrow  cylinder  on  each  side  of 
the  cambium  is  alive  and  functional  in  conduction. 
The  great  mass  of  the  tissue  of  the  trunk  is  dead  (heart- 
wood  and  outer  bark),  though  it  may  be  useful  to  the 
tree — the  heartwood  giving  extra  support,  the  outer 
bark  efficient  protection. 

The  bark  of  trees,  as  is  well  known,  varies  very  much 
in  appearance.  Some,  like  the  plane  tree  and  the 
"  paper  birch,"  have  smooth  bark,  which  scales  off  in 
uniform  flat  thin  layers.  This  is  because  the  phellogens, 
and  consequently  the  layers  of  cork  which  they  produce, 
are  uniform  and  parallel.  Others,  such  as  the  common 
oak,  the  elm  and  the  pine,  are  rugged,  the  surface  being 


USES   OF   CORK  341 

very  uneven  because  the  bark  scales  off  in  chunks. 
This  is  because  the  courses  of  the  phellogens  are  irregu- 
lar and  curved,  intersecting  one  another.  The  actual 
nature  of  the  cork  formed  also  varies  in  different  trees, 
sometimes  being  hard  and  woody,  while  in  other  cases 
the  corky  substances  of  the  cell  walls  is  very  pure, 
giving  an  elastic  character  to  the  bark. 

Uses  of  Cork. — The  "  cork  "  of  ordinary  life  is  obtained 
from  the  bark  of  the  cork  oak  (Quercus  suber) ,  an  ever- 
green oak  native  in  the  Mediterranean  region.  The  first- 
formed  bark  is  rough,  but  after  this  has  been  removed  the 
bark  subsequently  formed  is  very  uniform  in  structure, 
and  the  corky  substance  of  the  cell  walls  very  pure. 
This  bark  is  removed  in  sheets  at  intervals  of  several 
years,  boiled  in  vats  to  soften  it,  pressed  flat,  and  then 
cut  up  as  required.  The  dark  streaks  seen  in  cork  are 
the  lenticles,  composed  of  loose  powdery  cells,  which 
are  continuously  formed  by  the  phellogen,  so  that  they 
penetrate  the  whole  thickness  of  the  bark.  Bottle 
corks  are  always  cut  at  right  angles  to  the  course  of 
the  lenticels,  i.e.  parallel  to  the  surface  of  the  bark, 
so  that  solid  cork  intervenes  between  the  lenticels  in 
the  length  of  the  cork.  If  they  were  cut  the  other  way 
so  that  the  lenticels  ran  through  them  from  end  to  end, 
the  cork  would  not  be  gas  and  water  tight.  Bungs, 
on  the  other  hand,  which  are  not  intended  to  be  gas- 
tight,  are  stamped  straight  out  of  the  cork  sheet, 
so  that  the  lenticels  run  from  top  to  bottom  of  the 
bung. 

Cork  tissue  is  not  only  gas  and  water  tight,  it  is 
also  a  very  slow  conductor  of  heat.  Thus  a  jacket  of 
cork  prevents  the  living  tissues  in  the  interior  of  a 
tree-trunk  from  being  heated  or  cooled  too  quickly 
with  external  changes  of  temperature.  This  character, 


342  THE   WOODY  STEM 

together  with  its  lightness,  makes  it  useful  to  man  for 
a  great  variety  of  purposes. 


PRACTICAL  WORK. 

(1)  Make  a  sketch  of  a  winter  branch  of  horse-chestnut  (or 
beech)    showing    several  years'   growth.     Mark  terminal    buds, 
lateral  buds  (distinguishing  dormant  buds)  covered  with  winter  bud 
scales,  leaf  scars,  bud-scale  scars,  lenticels  ;  also  (in  horse-chestnut), 
if  present,   the  scar  of  a  fallen  inflorescence.     Mark  also  the 
parts  of  the  branch  originally  formed  in  each  year,  beginning  with 
last  season  and  working  backwards.     Dissect  a  large  terminal 
bud. 

(2)  Make  a  diagrammatic  sketch  showing  the  structure  of  any 
sprouting  bud   (sycamore  is  very  suitable,  but  almost  anything 
will  do)  after  the  first  foliage  leaves  are  clear  of  the  bud  scales, 
marking  winter  bud  scales,  foliage   leaves,  elongating   internodes 
and  terminal  bud.     [Opening  buds  preserved  in  spirit  should  be 
provided  if  fresh  material  is  unavailable  owing  to  the  season.] 

(3)  Examine  the  transverse  section  of  a  woody  twig  about 
three  years  old,  first  with  a  hand  lens,  then  with  the  low  power 
of  the  microscope.     [Sycamore  is  again  suitable,   but  elm  or 
lime  will  do  equally  well.]    Draw  a  diagram  under  the  low  power, 
showing — (a)  the  cork  ;  (b)  the  outer  cortex  ;  (c)  the  inner  cortex  ; 
(d)  the  pericycle  ;  (e)  the  secondary  phloem  ;   (/)  the  cambium  ; 
(g)  the  secondary  xylem  with  the  annual  rings,  and  in  each  the 
spring  and  autumn  wood  ;  (h)  the  principal  secondary  rays  ;  (i)  the 
primary  xylems  ;  (_;')  the  pith. 

(4)  Examine  the  branch  of   Portugal  Laurel  that  has  been 
kept  with  its  end  dipping  into  red  ink,  or  a  watery  solution  of 
eosin.     Split  it  longitudinally  and   note  that  the  red  solution 
has  eosin  in  the  wood  only. 

(5)  Make  diagrammatic  sketches  of  the  three  faces  (transverse, 
radial  and  tangential)  of  the  segment  of  oak  trunk  provided, 
showing — 

On  the  transverse  section  the  annual  rings,  the  large  spring 
vessels  which  appear  as  small  holes  in  the  wood,  the  masses  of 
fibres  making  up  the  great  bulk  of  the  wood,  the  broad  secondary 
rays  crossing  the  cambium  and  extending  through  the  functional 
living  phloem  of  the  inner  bark  (yellow  brown)  into  the  dead 
phloem  (red  brown)  cut  off  between  the  dark  layers  of  cork  of 
the  outer  bark. 

On  the  radial  face  the  spring  vessels  appear  as  grooves  in  the 


PRACTICAL  WORK  343 

wood  and  the  rays  as  "  silver  grain,"  because  their  course  is 
sinuous.  On  the  tangential  face  the  rays  are  seen  in  their  lens- 
shaped  transverse  section,  and  their  vertical  extent  can  be 
determined. 

Note  on  the  transverse  and  radial  faces  the  distinction  between 
heartwood  and  sapwood. 

(6)  Draw  under  the  high  power  a  small  portion  of  a  cross- 
section  of  the  outer  tissue  of  the  stem  of  a  woody  plant  during 
its  first  summer's  growth  to  show  the  origin  of  the  phellogen 
and  the  development  of  cork  [Ailanthus  is  suitable].     Compare 
the  remaining  tissues  of  the  stem  with  (3). 

(7)  Examine  the  piece  of  willow  branch  provided.     Observe 
the  lenticels  on  the  surface.     Peel  the  branch  and  note  that  the 
bark  separates  along  the  line  of  the  cambium. 


CHAPTER   XXI 

THE    FLOWER 

A  FLOWER  is  a  shoot,  or  the  termination  of  a  shoot, 
whose  leaves  (floral  leaves)  are  specially  modified  in 
different  ways.  The  upper  (inner)  leaves  produce  the 
reproductive  cells,  the  lower  (outer)  ones  serve  to 
protect  the  flower  bud  and  (usually)  to  render  the  flower 
conspicuous.  The  apical  meristem  of  the  flower  bud 
ceases  its  activity  with  the  production  of  the  floral 
leaves,  so  that  terminal  growth  in  length  of  the  floral 
shoot  is  at  an  end. 

The  seed  plants  are  heterosporous,  like  Selaginella 
(p.  251),  and  while  one  set  of  floral  leaves  bears  micro- 
sporangia  containing  microspores,  another  set  bears 
megasporangia,  each  megasporangium  normally  con- 
taining one  megaspore.  The  prothallus  produced  by 
each  is  greatly  reduced  and  is  formed  together  with  the 
gametes  inside  the  spore.  One  of  the  essential  characters 
of  seed  plants  is  the  means  by  which  the  gametes 
are  brought  together.  The  microspores  are  carried  by 
external  agency,  usually  insects  or  the  wind,  to  a 
special  part  of  the  floral  leaf  bearing  the  megasporangia. 
There  the  microspores  germinate,  each  putting  out  a 
germ  tube  which,  carrying  the  male  gametes,  grows 
towards  and  penetrates  the  megasporangium,  setting 
free  the  male  gametes  inside  the  megaspore,  in  the 
immediate  neighbourhood  of  the  female  gamete  or  egg. 
Conjugation  thus  takes  place  ijiside  the  megaspore. 


PARTS   OF   THE   FLOWER  345 

The  flowers  of  some  kinds  of  seed  plants  are  of  two 
kinds,  the  one  producing  microsporangia  alone  ("  male  " 
flowers),  the  other  megasporangia  alone  ("  female " 
flowers)  ;  but  most  seed  plants  have  flowers  producing 
both  kinds  of  sporangia,  though  on  separate  floral  leaves, 
and  these  are  called  "  hermaphrodite  "  flowers. 

Parts  of  the  Flower. — The  flower  is  a  condensed 
shoot  in  which  the  internodes  have  elongated  very  little,  if 
at  all,  and  the  crowded  nodes  on  which  the  floral  leaves 
are  borne  are  together  called  the  receptacle  (Fig.  57). 
The  lower  part  of  the  same  axis  continuous  with  the 
receptacle  is  often  bare  of  leaves,  and  is  called  the 
peduncle  if  the  flowers  are  borne  singly  arising  from 
vegetative  shoots,  the  pedicel  if  it  belongs  to  a  system 
of  branches  (inflorescence] ,  each  bearing  a  flower.  Leaves 
in  the  axils  of  which  flowers  are  borne  or  themselves 
borne  on  peduncle  or  pedicel  and  somewhat  different 
from  the  foliage  leaves  are  called  bracts. 

Perianth. — The  floral  leaves  themselves  are  arranged 
in  whorls  (a  whorl  is  a  circle  of  leaves  arising  at  the 
same  level  on  the  axis)  one  above  the  other.  The 
outer  (lowest)  leaves  form  the  perianth,  which  very 
commonly  consists  of  two  whorls,  the  calyx,  whose 
leaves,  the  sepals,  are  often  green  and  rather  like  small 
simple  foliage  leaves,  and  the  corolla,  whose  leaves,  the 
petals,  are  generally  larger  and  more  or  less  brightly 
coloured.  Sometimes,  however,  the  perianth  consists 
of  one  whorl  only,  or  of  two  whorls  whose  leaves  are 
alike.  In  the  flower  bud  the  perianth  leaves  are  curved 
inwards,  enclosing  the  two  inner  (upper)  whorls  of 
floral  leaves  which  bear  the  two  kinds  of  sporangia. 

Stamens  and  Carpels. — The  first  (lower)  of  these  is 
called  the  andrcecium,1  the  individual  leaves  the  stamens. 

1  Greek  dvrjp,  avdpoq  (aner,  andros),  a  man,  because  the  male  gametes 
are  formed  in  the  microspores  produced  by  the  stamens. 


346  THE   FLOWER 

Each  stamen  has  a  stalk,  the  filament,  and  bears 
(usually  two)  pairs  of  microsporangia,  which  are 
known  as  pollen  sacs,  together  forming  the  anther  or 
head  of  the  stamen.  When  ripe  the  pollen  sacs  are  full 


FIG.  57. — Diagram  showing  the  parts  of  a  flower. 

of  pollen  grains  (microspores)  (Fig.  58,  A).  The 
uppermost  whorl  of  floral  leaves,  occupying  the  centre 
of  the  flower,  is  called  the  gynaecium,1  and  its  leaves  the 
carpels.^  A  carpel  is  a  folded  leaf  with  its  margins 

1  Greek,  -ywrj,  ywaiKos  (gune,  gunaikos),  a  woman,  because  the 
female  gametes  are  formed  in  the  megaspores. 

»  Latin  carpellum,  a  little  fruit,  from  Greek  Kaptroq,  a  fruit,  because 
the  carpels  later  on  grow  into  the  fruit. 


ANTHER,    POLLEN   AND    POLLEN   TUBES  347 


D 


FIG.  58. — Development  of  pollen  (microspores) .  A,  cross-section  of 
anther  showing  four  pollen  sacs  (microsporangia),  one  full  of 
pollen  grains  (microspores)  ;  also  fibrous  layer  of  wall  and  vascular 
bundle.  B,  cross  section  of  opened  anther,  with  the  walls  of 
the  pollen  sacs  turned  back.  The  dotted  lines  show  their  original 
position  and  the  arrows  the  direction  of  movement  when  they 
break  open.  C,  pollen  grain  with  nucleus  (right)  and  "  genera- 
tion cell,"  which  represents  the  "  male  prothallus  "  (left).  D, 
germination  of  pollen  grain.  The  nucleus  has  entered  the  tube 
and  is  followed  by  the  "  generative  cell."  E,  growing  apex  of 
pollen  tube  with  nucleus  and  two  male  gametes  into  which  the 
generative  cell  has  divided.  F,  part  of  stigma  with  germinating 
pollen  grains,  their  tubes  pushing  down  between  the  loose  stig- 
matic  cells. 


348  THE  FLOWER 

coherent  so  as  to  form  a  closed  bag-like  structure,  the 
ovary  (Figs.  57 >  59 >  A).  The  free  end  of  the  leaf  forms 
a  structure,  which,  when  mature,  lacks  the  characteristic 
shoot  epidermis,  bearing  on  its  surface  papillae  or  hairs 
which  secrete  a  sugary  solution.  This  is  called 
the  stigma  (Fig.  57),  and  is  the  organ  which  receives  the 
pollen  grains.  The  stigma  is  often  raised  above 
the  ovary  on  a  more  or  less  hollow  stalk,  the  style.  On 
the  thickened  margins  of  the  carpellary  leaf  (placenta) 
inside  the  cavity  of  the  ovary  are  the  ovules  (Fig.  59,  A). 
Each  ovule  (Fig.  59,  B)  is  a  megasporangium  covered 
by  two  coats,  each  composed  of  one  or  more  layers  of 
cells,  and  with  a  body  (nucellus)  consisting  of  an  ovoid 
mass  of  cells,  of  which  one,  the  megaspore,  early  becomes 
much  larger  than  the  rest,  and  when  mature  fills  much 
of  the  space  within  the  nucellus,  one  end  lying  close 
to  the  surface  of  the  free  end  of  the  nucellus,  just  below 
a  pore  (micropyle]  left  by  the  incomplete  closure  of  the 
coats  of  the  ovule.  During  development  the  ovule 
generally  turns  completely  round  upon  itself,  i.e.  through 
an  angle  of  180  degrees,  so  that  the  free  end  comes  to 
point  towards  the  placenta  on  which  the  ovule  is  inserted. 
In  a  few  cases  the  ovule  does  not  turn,  and  the  free  end 
points  away  from  the  placenta. 

Development  of  Gametes. — The  pollen  grain  or 
microspore  when  ripe  consists  of  a  cell  with  a  thick 
cutinised  outer  wall,  often  covered  with  projections 
so  that  the  surface  is  rough,  and  a  thinner  internal 
wall  consisting  of  cellulose.  One  or  more  interruptions 
in  the  thick  outer  wall  leave  thin  places  covered  only 
by  the  thin  inner  wall.  The  cell  is  densely  filled  with 
cytoplasm  and  contains  a  large  conspicuous  nucleus. 
This  nucleus  divides  into  two  :  one  daughter  nucleus 
is  the  nucleus  of  the  ripe  grain  ;  the  other  is  somewhat 


DEVELOPMENT   OF   GAMETES 


349 

elongated  and  is  surrounded  by  a  little  cytoplasm,  so 
that  it  forms  a  little  naked  cell  within  the  pollen  grain 


ant. 


R 


FIG.  59. — The  ovule  and  fertilisation.  A,  cross-section  of  a  single 
free  carpel  containing  two  rows  of  ovules;  m.r.,  midrib  of  car- 
pellary  leaf;  pi.,  placenta  (thickened  margin  of  carpellary  leaf) 
bearing  row  of  ovules.  B,  ripe  ovule  (megasporangium)  in 
longitudinal  section;  v.b.,  vascular  bundle;  nuc.,  nucellus  (body 
of  ovule  =  wall  of  megasporangium);  o.c.,  outer  coat;  i.e.,  inner 
coat ;  micr.,  micropyle.  The  embryo  sac  (megaspore)  contains 
in  the  centre  the  secondary  nucleus  (sec.n.)  ;  at  the  micropylar 
end  the  two  synergidae  (syn.)  and  the  egg,  and  at  the  opposite 
end  the  three  antipodal  cells  (ant.).  C,  development  of  embryo 
sac ;  a-d,  division  of  primary  nucleus  to  form  eight  nuclei,  four 
at  each  end ;  e.,  formation  of  egg  apparatus,  group  of  antipodal 
cells  and  secondary  nucleus.  D,  embryo  sac  at  the  time  of 
fertilisation;  p.t.,  pollen  tube,  which  has  entered  the  upper  end 
of  the  sac  and  evacuated  the  two  male  gametes,  one  of  which 
is  seen  in  contact  with  the  nucleus  of  the  egg  (female  gamete) 
and  the  other  with  the  two  polar  nuclei,  which  in  this  case  have 
not  yet  fused  to  form  the  secondary  nucleus  of  the  sac.  Contact 
will  be  followed  by  fusion,  in  the  former  case  giving  the  zygote, 
in  the  latter  the  mother  nucleus  of  the  endosperm. 

cell  (Fig.  58,  C).     This  independent  cell  later  divides 
to  form  the  two  male  gametes  (E).      Thus  the  male 


350  THE  FLOWER 

prothallus  is  reduced  to  the  lowest  possible  terms,  for 
it  consists  of  nothing  but  the  single  mother  cell  of  the 
two  gametes  and  the  nucleus  of  the  grain  which  later 
functions  in  forming  the  pollen  tube. 

The  nucleus  of  the  megaspore  (embryo  sac)  divides 
by  three  successive  divisions  to  form  eight  nuclei 
(Fig.  59,  C,  a-e),  a  group  of  four  at  each  end  of  the 
sac.  Two  of  these  nuclei,  one  from  each  group  of  four, 
fuse  together  in  the  centre  to  form  the  secondary  nucleus 
of  the  sac,  the  other  three  of  each  group  remain  together 
and  accumulate  cytoplasm,  so  that  two  groups  of  three 
cells  are  formed  one  at  each  end  of  the  sac.  The 
three  cells  at  the  micropylar  end  of  the  sac  form  the 
egg  apparatus,  the  cell  farthest  from  the  micropyle 
being  the  egg  (female  gamete),  the  other  two  the 
synergids.1  The  three  cells  at  the  opposite  end  of  the 
sac  are  called  the  antipodal  cells  (Fig.  59,  D,  ant.). 

It  is  to  be  noticed  that  while  one  cell  only  of  the 
eight  normally  functions  as  a  female  gamete,  its  nucleus 
is  the  sister  of  a  nucleus  which  fuses  in  the  centre  of 
the  sac  with  a  corresponding  nucleus  from  the  antipodal 
group.  The  secondary  nucleus  so  formed  undergoes, 
as  we  shall  see  presently,  a  further  development. 
As  an  abnormality,  also,  one  of  the  synergidae  or  one 
of  the  antipodal  cells  may  fuse  with  a  male  gamete, 
thus  itself  acting  as  a  gamete.  We  must,  therefore, 
probably  regard  the  two  groups  of  cells  as  broods  of 
four  gametes,  only  one  of  the  eight  being  normally 
functional  as  a  sexual  gamete,  i.e.  fusing  with  a  male 
gamete,  though  two  others  normally  behave  in  a  gamete- 
like  way.  This  recalls  the  brood  of  eight  eggs  in 

1  Synergidae  (Greek  ovv,  with,  and  epyov,  work)  =  "  co-operators," 
because  these  two  cells  are  supposed  to  assist  in  directing  the  pollen 
tube  to  the  egg. 


POLLINATION  "AND   FERTILISATION  351 

Fucus,  of  which,  it  will  be  remembered,  all  are  func- 
tional in  some  species,  while  in  other  species  four, 
two,  or  only  one  are  functional  eggs. 

Pollination. — When  the  pollen  is  ripe  the  anthers 
open  in  various  ways.  Very  commonly  a  longitudinal 
split  runs  down  the  side  of  the  anther  between  the  two 
pollen  sacs  whose  walls  break  through  along  this  line 
and  fold  back,  exposing  the  pollen  grains  (Fig.  58,  B). 
The  split  is  actually  caused  by  the  contraction  on 
drying  up  of  the  wall  of  the  sac,  whose  cell  walls  have 
fibrous  thickenings,  the  tension  which  develops  eventually 
tearing  apart  the  weak  unthickened  tissue  between  the 
two  sacs  on  each  side  of  the  anther. 

The  pollen  grains  are  conveyed  to  the  stigma  in 
various  ways.  Sometimes  the  stigma  touches  the 
opened  anther,  rubbing  off  the  pollen  (self-pollination)  ; 
but  very  often  the  pollen  is  conveyed,  generally 
by  insects  or  the  wind,  to  the  stigma  of  another 
flower  of  the  same  species  (cross-pollination).  The 
stigma  is  papillose  and  sticky,  or  hairy,  so  that 
the  grains,  which  are  often  rough  coated,  readily 
stick  to  it. 

Germination  of  the  Pollen  Grains.  Fertilisation. — 
The  cells  of  the  stigma  secrete  a  sugary  solution  which 
is  absorbed  by  the  grains  lying  among  them,  and  these 
germinate,  a  tube  (pollen  tube)  being  pushed  out  from 
the  thin  spot  on  the  wall.  Into  this  tube  pass  first  the 
pollen  grain  nucleus,  and  then  the  independent  cell, 
which  divides  to  form  the  two  male  gametes  during 
the  growth  of  the  tube  (Fig.  58,  D,  E).  The  tube 
grows  into  the  stigma  (Fig.  58,  F)  and  down  through 
the  loose  tissue  of  the  style  till  its  tip  reaches  the  cavity 
of  the  ovary.  The  growing  tip  is  positively  chemotropic 
to  sugar,  or  some  other  substance  secreted  by  the  tissue 


352  THE   FLOWER 

in  the  region  of  the  micropyle,  which  the  tube  enters, 
then  pushing  between  the  cells  of  the  nucellus  and 
entering  the  embryo  sac.  Here  the  tip  bursts,  setting 
free  the  two  male  gametes.  These  are  elongated  and 
often  curved  or  slightly  twisted  spirally,  recalling  the 
spirally  twisted  male  gametes  of  the  lower  plants. 
They  appear  to  wriggle  actively  in  the  sac,  one  fusing 
with  the  egg,  the  other  with  the  secondary  nucleus  of 
the  sac  (Fig.  59,  D). 

The  zygote  or  "  fertilised  egg "  gives  rise  to  the 
embryo  of  the  new  plant,  the  secondary  nucleus  of 
the  sac,  to  which  three  nuclei  have  now  contributed, 
divides  to  form  a  new  tissue,  the  endosperm,  which  fills 
the  sac,  and  at  the  expense  of  which  the  embryo  grows. 
The  endosperm  nucleus  is  of  the  nature  of  a  zygote 
nucleus,  being  formed  by  the  fusion  of  (a)  the  sister 
nucleus  of  the  functional  female  gamete,  (b)  a  corre- 
sponding antipodal  nucleus,  (c)  a  male  gamete.  This 
"  triple  fusion  "  is  most  unusual,  and  leads  to  irregulari- 
ties in  division  of  the  endosperm  "  zygote  "  nucleus. 
The  endosperm  may  thus  be  regarded  as  an  "  abnorm- 
ally "  formed  embryo,  which  is  sacrificed  to  the  feeding 
of  the  normal  embryo. 

Varieties  of  Floral  Form  and  Structure. — The  variety 
of  form  and  structure  among  flowers  is  exceedingly 
great,  and  seed  plants  are  classified  largely  by  means 
of  these  differences.  Thus  the  floral  leaves  of  each 
kind,  particularly  the  stamens  and  carpels,  vary  from 
a  large  and  indefinite  number  (buttercup)  to  a  small 
and  definite  number  (apple,  cherry,  corncockle).  The 
floral  leaves  in  each  whorl  may  be  quite  free  and  separate 
from  one  another  x  (buttercup  and  Fig.  60,  A)  or  they 

1  This  is  expressed  by  the  prefix  apo-  (Greek  and,  away  from). 
Thus  the  buttercup  flower  is  apocarpous. 


INSERTION   OF   FLORAL   LEAVES   ON   RECEPTACLE      353 

may  be  joined  to  one  another  laterally I  (calyx  of 
corncockle,  carpels  in  Fig.  60,  B).  The  successive 
whorls  of  floral  leaves  may  arise  from  the  sides  of  a 
conical  receptacle  (Fig.  60,  A) ,  or  they  may  arise  from 


FIG.  60. — Diagrammatic  vertical  sections  illustrating  different  rela- 
tions of  the  receptacle  (black  throughout)  to  the  floral  whorls. 
A,  conical  receptacle  bearing  the  successive  whorls  on  its  sides 
(hypogynous  type),  carpels  separate.  B,  flat  receptacle,  carpels 
united  (syncarpous  gynaecium).  C,  basin-shaped  receptacle 
with  two  free  carpels  at  the  base,  the  remaining  whorls  on  the 
edge  of  the  basin  (perigynous  type).  D,  receptacle  fused  to  the 
sides  of  the  ovaries  (epigynous  type),  which  are  then  said  to  be 
"  inferior." 


a  flat  receptacle  (Fig.  60,  B),  or  the  receptacle  may 
be  cup-shaped  (Fig.  60,  C,  and  cherry),  with  the  sepals, 
petals  and  stamens  arising  from  the  edge  of  the  cup, 
and  the  carpels  from  its  bottom.  Finally,  the  sides  of 

1  Expressed  by  the  prefix  syn-  (Greek  ovv,  together).  The  corn- 
cockle flower  is  synsepalous.  The  gynaecium  in  Fig.  60,  B,  is  syn- 
carpous. 

23 


354  THE   FLOWER 

the  cup-shaped  receptacle  may  be  fused  with  the  walls 
of  the  carpels  (Fig.  60,  D  :  apple,  pear).  These 
different  relations  of  the  receptacle  to  the  various  whorls 
of  the  flower  are  expressed  by  the  terms  hypogynous, 
perigynous  and  epigynous,1  the  other  whorls  of  the 
flower  being  below  the  gynaecium  in  the  first  case, 
round  it  (i.e.  borne  on  the  edges  of  the  receptacular  cup) 
in  the  second,  and  above  or  on  it  in  the  third,  where 
the  sides  of  the  receptacle  are  fused  with  and  close  over 
the  walls  of  the  carpels. 

On  the  whole,  in  the  evolution  of  the  great  group 
of  flowering  plants,  the  flowers  with  separate  and  numer- 
ous floral  leaves  are  more  primitive,  those  with  few  and 
joined  floral  leaves  the  more  advanced  (the  stamens, 
having  thin  stalks,  generally  remain  separate  throughout, 
though  they  are  joined  to  one  another  in  a  few  cases). 
And  the  perigynous  and  epigynous  types  of  flower 
are  similarly  more  advanced  than  the  hypogynous. 
Especially  in  the  case  of  the  carpels,  as  we  pass  from  more 
primitive  to  more  advanced  flowers,  there  is  a  tendency 
for  the  carpels  (a)  to  decrease  in  number,  (b)  to  become 
fused,  and  (c]  to  become  enclosed  in  the  receptacle. 
The  ovary  is  said  to  be  inferior  in  this  last  (epigynous) 
type  of  flower  (apple,  narcissus). 

Besides  these  general  tendencies  in  the  evolution  of 
flowers  there  are  many  other  differences  depending 
on  the  relative  size,  shape  and  colour  of  the  different 
floral  leaves.  These  affect  particularly  the  mode  of 
pollination.  A  flower  has  to  be  regarded  as  a  whole, 
as  an  organ  which  not  only  produces  and  protects 
the  spores  and  gametes  but  is  also  so  constructed  as 
to  bring  about  the  conjugation  of  the  gametes  through 
the  preliminary  process  of  pollination.  In  many  cases 

1  Greek  VTTO,  Trepi  and  em,  below,  round  and  on. 


CROSS   POLLINATION  355 

the  flower  is  a  very  wonderful  and  perfect  mechanism 
for  securing  this  process. 

Cross-Pollination  by  Insects. — A  very  large  number 
of  flowers,  probably  the  great  majority,  are  adapted  to 
cross-pollination  by  insects.  Insects  of  different  kinds 
(mainly  beesi  flies,  butterflies  and  moths)  visit  flowers 
to  feed  on  the  nectar  or  on  the  pollen,  or  on  both,  or 
to  collect  them  (bees)  for  their  young.  While  in  the 


FIG.  61. — A,  ground  plan  of  a  flower  (floral  diagram)  showing  five 
free  sepals  (black),  five  alternating  free  petals,  ten  stamens  in 
two  alternating  whorls  of  five  each,  and  three  free  (apocarpous) 
carpels.  B,  cross-sections  of  syncarpous  ovaries  of  three  carpels 
each  (a,  b,  c),  showing  different  types  of  placentation  :  axile 
(note  correspondence  of  placentae  with  those  of  the  carpels  in 
A.),  free  central  (the  infolded  carpel  walls  have  disappeared,  leaving 
one  central  placental  column),  and  parietal  (carpels  not  infolded, 
margins  joined  to  form  placentae  on  inside  of  outer  wall). 

flower  they  brush  against  the  ripe  anthers,  and  the 
pollen  grains  stick  to  their  hairy  bodies.  On  visiting 
another  flower  of  the  same  species — and  bees  especially 
often  keep  to  one  kind  of  flower  on  one  journey — the 
insect  may  brush  against  the  stigma  and  rub  off  the 
grains.  The  positions  of  the  ripe  anthers  and  ripe 
stigmas  in  the  flower  are  generally  such  that  this  probably 
or  even  inevitably  happens. 

The   petals   of    insect-pollinated    flowers   are   often 
large,  brightly  coloured  and  conspicuous,  so  that  the 


356  THE   FLOWER 

larger  insects  can  see  them  from  a  distance.  The 
flower  often  has  nectaries,  little  masses  of  gland  cells 
which  secrete  a  sugary  solution,  and  this  nectar  is 
the  main  attraction  of  many  flowers  to  nectar-eating 
insects.  In  the  case  of  butterflies  and  moths  it  is  the 
sole  attraction.  The  ripe  anthers  and  ripe  stigmas  are 
commonly  held  in  such  a  position  that  the  insect  has 
to  brush  past  them  in  reaching  the  nectary. 

A  very  common  arrangement  is  that  in  which  the 
anthers  ripen  first,  and  the  filaments  bend  so  as  to 
bring  the  anthers  into  the  path  of  the  insect,  while 
later  on  the  stigmas  ripen  and  bend  into  the  same 
position.  Such  a  flower  is  called  protandrous.1  An 
insect  visiting  the  flower  in  the  first  stage  will  carry 
away  pollen,  and  this  will  be  rubbed  on  to  the  stigmas 
directly  it  visits  a  flower  in  the  second  stage,  since 
the  insect  will  brush  the  anthers  and  stigmas  in  the 
two  flowers  with  the  same  part  of  its  body. 

Thus  in  the  buttercup  the  nectaries  are  on  the  inner 
sides  of  the  petals  near  their  bases.  When  the  petals 
first  open  the  centre  of  the  flower  is  occupied  by  a  mass 
of  anthers  concealing  the  undeveloped  stigmas.  The 
first  anthers  to  open  are  those  on  the  outside,  next 
the  petals,  and  the  ripening  of  the  anthers  progresses 
gradually  inwards  to  the  innermost  ones,  the  ripe 
anthers  bending  outwards.  This  is  the  "  male  stage  " 
of  the  flower.  If  a  large  insect  visits  the  flower  it  alights 
in  the  centre  on  the  mass  of  anthers  and  pushes  its 
head  down  between  these  and  the  petals  to  get  at  the 
nectar.  In  doing  so  pollen  will  adhere  to  the  lower 
side  of  its  head  or  body.  By  the  time  the  last  anthers 
have  opened  the  carpels  have  grown  up  in  the  middle 


1  Greek  Trp&ros,  first,  and  a.vr\p,    dvdpos,  man,  because   the   male 
elements  (pollen)  ripen  first. 


CROSS   POLLINATION  357 

of  the  flower  and  their  stigmas  are  ripe  (incomplete 
protandry).  The  same  insect  alighting  on  the  centre 
of  the  flower  in  this  (the  "  female  ")  stage  will  brush 
off  on  the  stigmas  any  pollen  that  may  be  adhering  to 
the  lower  side  of  its  body  from  a  previous  visit  to  a 
flower  in  the  male  stage,  thus  effecting  cross-pollina- 
tion. A  small  insect,  on  the  other  hand,  may  alight 
anywhere,  on  the  petals  or  among  the  anthers,  and 
will  crawl  down  to  the  nectary  or  wander  about  eating 
pollen,  and  it  will  only  touch  the  stigmas  in  the  female 
stage  by  chance,  if  at  all.  The  majority  of  large  con- 
spicuous flowers  are  pollinated  mainly  by  large  insects. 
In  the  absence  of  insect  visitors,  which  do  not  visit 
flowers  in  cold  dull  weather,  the  stigmas  of  the  butter- 
cup eventually  curl  out  far  enough  to  come  into 
contact  with  the  innermost  anthers,  which  still  probably 
have  some  pollen  adhering  to  them,  and  thus  effect 
self-pollination . 

In  the  corncockle  (Lychnis  githago)  and  the  pinks 
(Dianthus)  protandry  is  complete,  so  that  self-pollina- 
tion is  impossible.  The  anthers  lie  on  the  platform 
provided  by  the  flat  limbs  of  the  petals  on  which  the 
insect  must  alight.  At  this  time  the  unripe  stigmas 
are  concealed  in  the  narrow  tube  of  the  flower.  Then 
the  anthers  fall  off  and  the  stigmas  grow  up  and  lie 
on  the  platform  in  exactly  the  same  position  that  the 
anthers  previously  occupied. 

In  other  flowers  which  are  not  protandrous  the  stigmas 
are  generally  held  well  in  advance  of  the  anthers,  so 
that  they  are  the  first  objects  the  insect  meets  in  entering 
the  flower,  and  any  pollen  it  already  bears  will  tend  to 
be  rubbed  off  on  them.  On  pushing  further  into 
the  flower  the  anthers  are  encountered. 

Some  flowers  are  so  nicely  adjusted  to  the  structure 


35§  THE  FLOWER 

and  habits  of  particular  insects  that  they  are  only  cross- 
pollinated  when  they  receive  visits  from  these  insects, 
and  remain  sterile,  setting  no  seed,  in  their  absence. 

Inconspicuous  flowers  are  often  adapted  for  wind- 
pollination  (grasses,  sedges,  most  catkin-bearing  trees, 
pines  and  firs,  etc.).  Wind-pollinated  flowers  produce 
pollen  in  large  quantities,  and  this  is  smooth  and  dust- 
like,  not  rough  or  sticky,  like  the  pollen  grains  usually 
conveyed  by  insects.  A  great  deal  is  wasted  because 
its  carriage  to  the  stigmas  of  another  flower  of  the 
same  species  is  a  matter  of  pure  chance.  The  stigmatic 
surfaces  are  generally  large,  and  this  of  course  increases 
the  chance  of  some  of  the  right  pollen  being  caught. 

Darwin  showed  that  the  offspring  of  crossing  are, 
on  the  whole,  and  with  certain  exceptions,  more  vigorous 
than  the  offspring  of  self-pollination,  when  individuals 
arising  from  seed  produced  in  the  two  ways  are  grown 
side  by  side.  Thus  variations  in  the  flower  tending 
to  secure  crossing  will  tend  to  be  fixed  and  perpetuated 
because  the  offspring  of  flowers  with  such  variations 
will  tend  to  survive  more  often  than  those  in  which 
self-pollination  occurred.  It  seems  to  be  only  in  this 
way  that  we  can  explain  the  origin  and  fixation  of  the 
various  beautiful  and  often  astonishingly  accurate 
mechanisms  which  bring  about  crossing  in  flowers. 

Many  inconspicuous  flowers  (chickweed,  the  small 
field  speedwells,  etc.)  are,  however,  habitually  self- 
pollinated,  and  these  perpetuate  themselves  indefinitely 
with  perfect  success.  Many  garden  and  field  crops 
(green  peas,  wheat)  are  in  the  same  position.  There 
is  no  evidence  that  continued  "  self -fertilisation  "  is 
in  any  way  harmful  to  the  race,  though  a  chance  cross 
in  an  habitually  self-fertilised  species  will  often  result 
in  a  distinct  increase  in  the  vigour  of  the  offspring. 


THE    FLOWER  359 

PRACTICAL  WORK. 

(1)  Cut  a  flower   of   buttercup  T    (Ranunculus)  longitudinally 
exactly  down  the  middle  with  a  razor  or  very  sharp  knife,  and 
draw  the  cut  surface  on  an  enlarged  scale,  showing  (a)  receptacle, 
(b)  calyx  of  sepals,  (c)  corolla  of  petals,  with  (d)  nectaries,  (e)  stamens, 
(/)  carpels. 

(2)  Draw  under  a  lens  a  single  stamen  showing  (a]  the  filament, 
(b)  the  anther  with  two  pairs  of  pollen  sacs,   (c)   the  connective 
(continuation  of  the  filament  between  the  pairs  of  pollen  sacs), 
(d)  the  line  of  dehiscence  of  the  anthers.     Draw  also  a  side  view 
of  a  carpel  showing  ovary  and  stigma.     Mount  a  stamen  and  a 
carpel  in  a  drop  of  dilute  glycerine  on  a  slide  and  examine  with 
the  low  power. 

(3)  Make   a  diagram  of  a  section  through   a  mature  anther, 
showing  the  four  pollen  sacs,  in  some  of  which  two  nuclei  may 
be  seen,  the  connective  traversed  by  a  vascular  bundle,  and  the 
"  fibrous  layer  "  of  cells  between  the  epidermis  and  the  cavity 
of  each  pollen  sac.     The  cells  of  this  bear  rib  thickenings  on  their 
walls,  and  it  is  the  tension  developed  in  this  layer  that  splits 
the  anther  open. 

Compare  the  section  through  a  dehisced  anther. 

(4)  Remove  a  carpel  from  the  old  flower  or  young  fruit  of 
Caltha  (Marsh  Marigold),  draw  the  side  view  showing  ovary  and 
stigma,  and  then  split  the  ovary  open  so  as  to  show  the  two 
rows  of  ovules  attached  to  the  inner  edges  (towards  the  centre 
of  the  flower),  which  are  the  joined  margins  (placentae)  of  the 
carpellary  leaf. 

(5)  Under  a  low  power  draw  a  diagram  of  the  transverse  section 
of  the  carpel  of  Caltha  or  Aquilegia  (Columbine),  showing  the 
closed  ovary  of  the  carpellary  leaf  with  its  midrib,  and,  on  the 
opposite  side,  its  joined  margins  (placentae)  to  which  the  ovules 
are   attached.     The  section  should  pass  longitudinally  through 
the    centre   of    one  ovule,  and  the  side  of  the  next  belonging 
to  the  other  row,  since  the  ovules  of  the  two  rows  alternate. 

(6)  In  a  section  passing  through  the  centre  of  an  ovule  identify 
and  draw  carefully  under  the  high  power  (a)  the  coats  of  the  ovule 
(identify  if  possible  the  micropyle,  which  is  very  narrow  and  may 
not  be  traversed) ,  (b)  the  nucellus,  (c)  the  embryosac  with  vacuolated 
cytoplasm  and  conspicuous  secondary  nucleus,  (d)  the  antipodal 

1  If  the  buttercup  cannot  be  obtained  fresh,  it  is  well  to  examine 
first  any  fairly  large  fresh  flower  that  can  be|fobtained,  draw  a  median 
section  through  it,  and  then  compare  with  the  preserved  buttercup 
flower. 


360  THE   FLOWER 

cells  (also  very  conspicuous),  (e)  the  egg  cell  and  the  two  synergidtz. 
Note  that  the  body  of  the  ovule  is  turned  round  so  that  the 
micropylar  end  faces  towards  the  placenta.  Note  also  the  vascular 
bundle  running  from  the  placenta  up  the  stalk  of  the  ovule 
to  the  base  of  the  nucellus. 

(7)  Compare  the  "  male  "  and  "  female  "  stages  of  the  corn- 
cockle (Lychnis),  pink  (Dianthus),  or  other  strongly  protandrous 
flower.     Make  careful  drawings  of  longitudinal  sections  through 
the  flower  in  each  stage.     [In  Lychnis  or  Dianthus  show  the  tubular 
calyx  (synsepalous),  the  separate  petals,  each  with  "  claw  "  and 
"  limb  "  (the  limbs  forming  the  alighting  platform  for  insects), 
the   ten   stamens,    and   the    (five)    carpels   joined    (syncarpous) 
to   form    a    single  ovary  with    free  central  placenta,    and    five 
stigmas.] 

(8)  Examine  a   flower   of   the   cherry,    and   draw   a   median 
longitudinal  section  through  it,  noting  especially  the  cup-shaped 
receptacle  with  sepals,  petals  and  stamens  borne  on  its  edge,  and 
the  single  carpel  at  the  base  of  the  cup,  with  a  long  style,  which 
projects  above  the  opening  and  bears  a  flat  stigma  at  its  summit. 


CHAPTER   XXII 

THE  FRUIT 

JUST  as  the  development  of  the  flower  culminates  with 
the  ripening  of  the  spores  and  the  production  of  the 
gametes  followed  by  fertilization,  so  the  development 
of  the  fruit  culminates  with  the  ripening  of  the  seeds, 
which  are  the  ovules  after  the  zygote  (fertilised  egg) 
has  developed  into  the  embryo  of  the  new  plant. 

Development  of  Seed  from  Ovule. — After  fertilisation 
the  zygote  divides  and  forms  a  (usually  spherical) 
embryonal  cell  towards  the  centre  of  the  embryo  sac, 
and  a  stalk  (suspensor)  connecting  this  with  a  basal 
cell,  which  remains  attached  to  the  micropylar  end  of 
the  sac  (Fig.  62,  A).  The  cells  of  the  suspensor  divide 
at  right  angles  to  its  length  and  elongate,  pushing  the 
embryonal  cell  down  into  the  endosperm  I  tissue  which 
is  formed  by  the  rapid  division  of  the  secondary  nucleus, 
food  substances  being  poured  into  the  sac  through  the 
vascular  bundle.  Between  the  numerous  nuclei  which 
arise  from  this,  cell  walls  appear,  thus  filling  the  sac 
with  parenchymatous  tissue. 

The  embryonal  cell  now  divides,  and  the  mass  of 
cells  to  which  it  gives  rise  forms  the  embryo  proper,  the 
suspensor  ceasing  to  grow  (Fig.  62,  B).  In  a  dicotyle- 
dinous  plant  (the  majority  of  the  flowering  plants) 
two  rounded  projections  arise  on  the  free  end  of  the 
embryo  (Fig.  62,  C),  and  these  develop  into  the  two 

1  From  Greek  evdov  and  OTrepua,  "  inside  the  seed." 
361 


362 


THE   FRUIT 


FIG.  62. — Development  of  embryo.  A,  embryo  sac  shortly  after 
fertilisation.  The  secondary  nucleus  has  divided  many  times 
and  the  sac  is  rapidly  filling  with  the  endosperm  tissue  ;  food 
is  coming  in  through  the  vascular  bundle  of  the  ovule  (arrows) .  The 
fertilised  egg  (zygote)  has  divided  to  form  the  suspensor  and  the 
embryonal  cell.  B,  sac  full  of  endosperm,  into  which  the  embryo 
is  being  pushed  by  the  elongation  of  the  suspensor.  C,  growth 
of  embryo  (cells  not  shown)  :  the  free  end  develops  two  lobes  (the 
rudiments  of  the  cotyledons).  D,  older  embryo  ;  cot.,  cotyledons  ; 
hyp.,  hypocotyl;  r,  radicle  ;  susp.,  suspensor.  E,  ripe  endospermic 
seed  showing  embryo  with  two  thin  cotyledons  closely  appressed 
with  epicotyledonary  bud  (epic.)  between,  hypocotyl  (hyp.), 
radicle  (rad.)  ;  also  endosperm,  perisperm  (per.),  testa,  and 
micropyle.  Note  B-E  are  drawn  on  progressively  smaller  scales, 
the  size  of  the  seed  and  embryo  having  very  greatly  increased. 


DEVELOPMENT   OF   EMBRYO  363 

cotyledons,  which  are  the  first  two  leaves  of  the  plant 
(Fig.  62,  D,  E).  Between  them  the  terminal  bud  of  the 
primary  shoot  axis  (epicotyl)  is  formed.  At  the  other 
end  of  the  embryo,  where  it  joins  the  suspensor,  the 
primary  root  is  formed,  the  apical  meristem  just 
inside  the  surface  layer  of  cells  which  form  the  first 
root  cap. 

In  the  monocotyledons  z  the  free  end  of  the  embryo 
forms  the  single  cotyledon,  and  the  epicotyledonary 
bud  is  developed  laterally.  The  part  of  the  primary 
axis  between  the  cotyledons  and  the  primary  root  is 
the  hypocotyl. 

The  embryos  of  different  species  vary  very  much  in 
the  stage  of  development  they  have  reached  by  the 
time  the  seed  is  ripe.  In  some  cases,  particularly  in 
small  seeds,  the  embryo  remains  in  a  very  rudimentary 
stage  of  development,  surrounded  by  the  endosperm. 
In  others  it  develops  within  the  seed,  not  only  large 
leaf-like  cotyledons,  but  an  epicotyledonary  bud  with 
the  rudiments  of  several  additional  leaves.  In  such 
large  well- developed  embryos  the  vascular  system  of 
cotyledons,  hypocotyl  and  primary  root  is  all  "  blocked 
out,"  so  that  the  various  tissues  can  be  clearly  recognised 
in  the  embryo,  though  the  cells  remain  small  and  without 
any  thickenings  on  their  walls  till  germination.  Very 
often  the  seed  grows  to  many  times  the  size  of  the 
ovule  at  fertilisation,  as  for  instance  in  the  bean.  This 
extensive  development  of  the  embryo  requires  of  course 
an  ample  supply  of  food,  which  is  brought  up  through 
the  vascular  bundles  of  the  placenta  and  of  the  ovule 
stalk  to  the  developing  seed.  The  embryo  itself 
grows  at  the  expense  of  food  absorbed  from  the  endo- 

1  Monocotyledons  include  the  grasses,  sedges,  lilies,  orchids,  palms, 
etc. 


364  THE   FRUIT 

sperm,  and  this  is  supplied  from  the  bundle  which 
terminates  at  the  base  of  the  nucellus. 

When  the  embryo  is  ripe  it  may  fill  the  whole  of  the 
space  within  the  embryo  sac,  or  it  may  still  be  sur- 
rounded by  a  mass  of  endosperm  (Fig.  62,  E)  (see  p.  362). 
The  nucellus  of  the  ovule  is  sometimes  represented  in 
the  seed  by  a  thin  layer  of  tissue  which  may  be  stored 
with  food  substance  (perisperm).  The  coat  (or  coats) 
of  the  ovule  becomes  differentiated  in  various  ways, 
their  cell  walls  generally  thickened  and  cutinised  or 
lignified,  to  form  the  seed  coat  or  testa. 

It  will  be  useful  here  to  summarise  the  corresponding 
structures  in  ovule  and  seed  : — 

The  ovule  becomes  the  seed. 

The  coat   (or  coats)   of  the  ovule  becomes  the  testa. 

The  nucellus  may  become  the  perisperm. 

The  tissue  formed  by  the  division  of  the  secondary 
nucleus  of  the  embryo  sac  forms  the  endosperm. 

The  zygote  becomes  the  embryo  (together  with  the 
suspensor — a  transitory  structure). 

Development  of  Fruit  from  Carpels,  etc. — While  the 
changes  described  above  are  taking  place  in  the  ovule, 
others  are  proceeding  in  the  rest  of  the  flower.  The  petals 
and  stamens  usually  fall  off  very  soon  after  fertilisation, 
while  the  calyx  is  often,  though  not  always,  persistent, 
and  sometimes  grows  considerably  in  size.  The  stigma 
and  style  fall  off  or  wither  away,  but  the  walls  of  the 
ovary  typically  develop  into  the  walls  of  the  fruit 
(pericarp),  keeping  pace  with,  or  even  outstripping,  the 
growth  of  the  seeds.  The  pericarp,  into  which  the 
ovary  wall  develops,  differs  very  much  in  different 
species,  being  sometimes  thin  and  membranous  (pea), 
sometimes  thick  and  woody  (hazel  nut),  sometimes 
soft  and  succulent  (raspberry,  tomato). 


STRUCTURE   OF   FRUITS  365 

In  popular  language  a  fruit  is  a  fleshy  envelope  which 
can  be  eaten  enclosing  seeds,  but  the  edible  part  is  not 
necessarily  formed  from  the  pericarp.  For  instance, 
the  fleshy  part  of  an  apple,  a  fig,  or  a  pineapple,  is  not 
pericarp  at  all.  The  botanical  conception  of  a  "  true  " 
fruit  (i.e.  the  structure  formed  from  the  carpels  alone 
by  the  time  the  seeds  are  ripe)  is  both  wider  and  narrower 
than  the  popular  conception.  Thus  it  includes  the 
bean  pod  and  the  coconut,  while  it  excludes  all  but 
the  "  pips  "  of  a  fig,  and  the  core  of  an  apple.  We 
may,  however,  conveniently  include  in  the  general  term 
"  fruit  "  all  the  structures  enclosing  the  ripe  seeds, 
whether  derived  from  pericarp  or  not. 

The  only  way  to  understand  the  nature  of  the  parts 
of  a  fruit  in  this  wide  sense  is  to  follow  their  develop- 
ment from  the  flower.  It  is  especially  the  receptacle 
that  very  frequently  takes  part  in  the  structure  of  the 
fruit.  This  is  necessarily  the  case  in  all  fruits  formed 
from  "  inferior  "  ovaries  (p.  354),  because  the  wall  of  the 
receptacle  is  here  fused  with  the  wall  of  the  carpels. 
The  wall  of  the  receptacle  and  the  wall  of  the  carpel 
may  together  form  quite  a  thin  membrane,  but,  on  the 
other  hand,  one  or  other  or  both  may  swell  up  and 
become  fleshy  in  the  fruit. 

The  cherry,  the  rose  and  the  apple  belong  to  the 
same  family  (Rosaceae,  the  rose  family),  and  they  illus- 
trate these  differences  very  well.  The  cherry,  as  we 
have  already  seen,  has  a  cup-shaped  receptacle  with 
a  single  free  carpel  at  the  base  of  the  cup.  It  is  the 
ovary  of  the  carpel  alone  which  forms  the  cherry.  The 
receptacle  does  not  grow  after  fertilisation,  and  is  soon 
flattened  out  by  the  great  growth  of  the  young  fruit — 
it  can  still  be  seen  as  a  little  disc  at  the  base  of  the  cherry 
where  the  stalk  joins  the  fruit.  The  wall  of  the  carpel 


366  THE  FRUIT 

becomes  differentiated  into  three  layers — the  "  skin," 
the  "  flesh  "  and  the  "  stone  "  :  the  "  kernel  "  is  the 
seed. 

In  the  rose  the  flower  is  perigynous  as  in  the  cherry, 
but  there  are  many  separate  carpels,  each  containing 
a  single  seed,  within  the  urn-shaped  receptacle,  beyond 
the  mouth  of  which  the  stigmas  proj  ect.  In  the  develop- 
ment of  the  fruit  the  carpels  themselves  do  not  increase 
very  much  in  size,  but  the  wall  of  the  receptacle  grows 
and  becomes  fleshy,  forming  the  well-known  red  "  hip  " 
or  rose  fruit. 

In  the  apple  and  pear  the  receptacle  in  the  flower 
has  very  much  the  same  shape  as  in  the  rose,  though  it 
is  less  elongated  ;  but  it  is  fused  with  the  walls  of  the 
five  carpels  enclosed  within  it.  Both  receptacle  and 
ovaries  increase  greatly  in  size  after  fertilisation,  the 
former  becoming  the  "  flesh  "  of  the  apple  and  the 
latter  the  "  core."  The  "  pips "  are  the  seeds.  A 
cross-section  of  an  apple  shows  that  the  ovaries  are 
separate  from  one  another  as  they  are  in  the  rose,  though 
they  are  embedded  in  the  flesh  of  the  receptacle. 

In  the  strawberry  the  receptacle,  instead  of  being 
hollow  and  covering  the  carpels,  is  convex,  and  in  the 
development  of  the  fruit  it  swells  and  becomes  succulent 
separating  the  one-seeded  carpels  ("  pips ")  by  its 
great  increase  in  surface,  over  which  the  carpels  are 
distributed.  The  style  of  each  carpel  may  still  be 
seen  attached  to  each  "  pip."  The  cinquefoils,  which 
are  close  allies  of  the  strawberry,  have  exactly  similar 
carpels,  but  the  receptacle  does  not  become  fleshy, 
so  that  the  carpels  are  still  crowded  together  in  fruit. 

In  the  mulberry,  which  is  derived  from  an  inflorescence 
(group  of  flowers),  it  is  the  perianth  leaves  (two  pairs) 
which  become  succulent,  enclosing  the  pip-like  ovary. 


INDEHISCENT   AND   DEHISCENT   FRUITS  367 

Sometimes  a  whole  inflorescence  develops  into  a 
single  fruit  (often  called  an  "  aggregate  fruit  ").  In 
the  fig,  for  instance,  the  axis  of  the  inflorescence  is 
concave,  bearing  the  crowded  flowers  on  its  inner  surface. 
In  fruit  the  axis  (common  receptacle  of  the  flowers) 
becomes  succulent,  the  ovaries  ("  pips  ")  remaining 
hard.  In  the  pineapple  the  whole  of  the  flowers  of 
the  inflorescence  (including  perianth  and  stamens), 
as  well  as  the  bases  of  the  bracts,  become  succulent. 
The  hard  tips  of  the  bracts  are  exposed  on  the  surface 
of  this  "  aggregate  "  fruit. 

Indehiscent  and  Dehiscent  Fruits.— Sometimes  the 
change  in  form  and  appearance  of  the  carpels  in  the 
passage  from  flower  to  fruit  is  very  slight,  as  in 
the  buttercup,  rose,  strawberry.  In  the  buttercup,  for 
instance,  the  carpels,  which  are  one-seeded,  scarcely 
grow  at  all,  but  their  walls  become  dry  and  membranous, 
changing  in  colour  from  green  to  brown.  When  ripe 
they  are  easily  detached  from  the  receptacle  and  are 
shaken  off  by  the  wind.  The  pericarp  softens  and 
decays  in  damp  soil,  and  when  the  seed  inside  germinates 
the  young  plant  pushes  through  its  remains.  This 
is  called  an  indehiscent  fruit,  because  the  pericarp  does 
not  open  when  the  seeds  are  ripe.  A  dry  one-seeded 
membranous  walled  fruit  "of  this  kind  is  called  an 
achene.  It  is  a  common  type,  not  only  in  the  butter- 
cup family  (Ranunculaceae),  as  in  buttercup,  anemone, 
clematis,  but  also  in  the  rose  family  (Rosacese),  where 
it  is  found  in  cinquefoil  (Potentilla),  herb-bennet  (Geum) 
and  others  ;  and  it  may,  as  we  have  seen,  be  associated 
with  a  succulent  receptacle  (rose,  strawberry).  Much 
the  same  type  of  fruit  is  found  in  the  great  family 
Compositse,  in  which  the  flowers  are  aggregated  in  close 
heads  (dandelion,  thistle),  and  the  outer  ones  are 


368  THE   FRUIT 

frequently  sterile,  with  a  conspicuous  one-sided  corolla, 
so  that  the  whole  flower-head  looks  like  a  single  flower 
(daisy).  Here  the  gynaecium  of  each  flower  consists 
of  two  united  carpels,  the  ovary  being  inferior.  Only 
one  carpel,  however,  with  one  seed,  comes  to  maturity. 
The  wall  of  the  ovary  really  consists  of  receptacle 
and  pericarp  fused,  but  since  it  is  membranous  and 
one-seeded  the  mature  fruit  is  for  practical  purposes 
an  achene,  though  it  is  derived  from  a bicarpellary  inferior 
ovary. 

A  nut  is  a  one-seeded  fruit  with  a  woody  pericarp. 
Here  again  it  may  be  derived  from  a  bicarpellary  inferior 
ovary,  as  in  the  common  hazel  nut  (Corylus). 

Dry  fruits  which  contain  many  seeds  are  practically 
always  dehiscent,  i.e.  they  open  to  let  out  the  ripe  seeds. 
A  simple  example  is  a  pea  or  bean  pod.  This  consists 
of  a  single  elongated  carpel  held  horizontally  in  the 
flower,  and  generally  more  or  less  flattened  laterally. 
The  lower  edge  is  the  midrib  of  the  carpellary  leaf, 
the  upper  edge  the  joined  margins  (placentae)  bearing 
the  seeds.  The  seeds  and  the  ovary  grow  very  con- 
siderably before  ripening,  multiplying  their  original 
size  many  times  over.  When  fully  grown  the  walls 
dry  and  split  along  both  edges,  the  seeds  being  easily 
detached  and  falling  to  the  ground.  Practically  all 
the  pea  family  (Papilionaceae)  have  this  type  of  fruit 
(legume).  The  marsh  marigold,  hellebore,  columbine, 
larkspur  and  monkshood — all  members  of  the  butter- 
cup family  (Ranunculaceae) — have  similar  fruits,  but 
they  have  several  separate  carpels  in  each  flower,  and 
the  fruit  splits  along  the  placental  margin  only  (follicle). 

Many  dry  dehiscent  fruits  are  formed  from  syncarpous 
gynaecia,  with  axile,  free  central  or  parietal  placentation 
— Fig.  6 1  (foxglove,  corncockle,  violet).  These  all  open  by 


DISPERSAL   OF   FRUITS   AND   SEEDS  369 

the  longitudinal  splitting  of  the  dry  capsule,  as  this  kind 
of  fruit  is  called.  In  other  cases  the  top  of  the  capsule 
comes  off  along  a  horizontal  line  (pimpernel) .  Sometimes 
the  capsule  opens  by  pores  in  the  wall  (poppy) ,  out  of 
which  the  very  small  seeds  are  shaken  when  the  stem 
is  swayed  by  the  wind,  by  which  they  may  be  carried 
a  considerable  distance  from  the  parent  plant. 

Methods  of  Fruit  and  Seed  Dispersal,  (i)  Dispersal 
by  Wind. — This  is  a  very  common  means  of  dispersal. 
Seeds  and  dry  one-seeded  fruits  vary  very  much  in  the 
distance  they  may  be  blown.  The  smaller  the  seed 
or  fruit  the  further  it  will  be  carried,  if  the  shape  and 
the  specific  gravity  are  the  same,  because  the  smaller 
the  seed  the  greater  the  ratio  of  surface  to  bulk.  Many 
small  seeds  are  carried  for  some  distance  in  this  way. 
On  the  other  hand,  a  large  seed  like  the  broad  bean 
is  scarcely  affected  by  the  wind  and  simply  drops  out 
of  the  pod.  If  a  seed  is  flat  instead  of  being  round, 
its  surface  will  be  greatly  increased,  and  it  will  blow 
much  further.  Many  seeds  and  one-seeded  fruits 
bear  outgrowths  from  the  testa  or  pericarp,  which  greatly 
increase  the  surface  on  which  the  wind  can  act.  These 
are  of  two  main  kinds — wings  and  hairs  (plumes). 

Winged  fruits  are  pretty  common.  A  well-known 
example  is  the  sycamore  fruit.  There  are  two  joined 
carpels  in  the  flower,  and  from  the  free  side  of 
each  a  curved  wing  grows  out.  When  the  fruit  is 
ripe  the  two  carpels  split  apart,  and  the  two  are  often 
detached  from  the  tree  separately.  The  shape  of  the 
wing  causes  the  one-seeded  "  mericarp "  (separated 
part  of  the  whole  fruit)  to  spin  as  it  falls,  and  it  is  thus 
more  likely  to  be  carried  away  by  the  wind.  Other 
examples  of  winged  one-seeded  fruits  are  those  of  the  ash 
("  keys  "),  the  birch,  and  the  hornbeam.  In  the  last  two 
24 


370  THE   FRUIT 

the  wings  are  formed  by  adherent  bracts.  Examples 
of  winged  seeds  are  those  of  the  pine  and  of  the  tropical 
genus  Zanonia. 

Examples  of  plumed  fruits  are  those  of  the  "  old 
man's  beard  "  (the  wild  clematis),  where  the  styles  are 
persistent  in  the  fruit  and  are  clothed  with  long  silky 
hairs,  and  those  of  the  dandelion,  groundsel,  thistle 
("  thistle  down "),  and  of  many  other  members  of 
the  Compositae.  Here  there  is  a  special  plumed  structure, 
the  pappus,  surmounting  the  indehiscent  one-seeded 
fruit.  Examples  of  plumed  seeds  are  those  of  the 
willows,  and  of  the  various  species  of  willowherb  (the 
riverside  "  codlins  and  cream,"  the  "  rosebay  "  willow- 
herb  of  open  woods,  and  the  other  species).  Here  the 
capsule  opens  and  sets  free  the  crowds  of  tiny  seeds 
which  drift  about,  floated  in  the  air  by  their  very  fine 
silky  hairs. 

(2)  Dispersal  by  Animals. — Some  indehiscent  fruits 
bear  hooks  which  catch  on  passing  animals  and  stick  to 
their  coats,  the  fruits  being  readily  pulled  off  the  plant. 
Brushed  off  later  on,  the  seed  may  germinate  at  a  distance 
from  the  parent  plant.  The  hooks  are  sometimes  out- 
growths of  the  pericarp  ("  cleavers,"  Galium  aparine), 
or  the  hook  is  formed  from  the  style  (herb-bennet, 
Geum  urbanum).  In  the  burdock  (Arctium),  a  member 
of  the  Compositae,  the  bracts  surrounding  the  flower 
head  are  hooked,  and  the  head  when  in  fruit  is  easily 
detached,  so  that  it  is  carried  off  as  a  whole,  and  the 
dry  one-seeded  fruits  drop  off.  All  these  fruits  very 
readily  stick  to  the  trousers  or  skirt  as  the  wearer 
brushes  against  the  plants. 

Animals  are  also  constantly  distributing  dry  fruits 
and  seeds  even  if  these  have  no  special  means  of  adhering 
to  them — carrying  them  about  in  the  crevices  of  their 


SUCCULENT  FRUITS  371 

bodies,  in  the  mud  adhering  to  their  feet  and  so  on. 
An  almost  incredible  number  of  different  seeds  and 
small  dry  fruits  may  often  be  found  in  the  pockets  and 
seams  of  the  clothes  as  well  as  on  the  boots  of  men  who 
spend  much  time  in  the  open  country. 

In  the  case  of  succulent  fruits  which  are  sought  and 
eaten  by  birds  and  mammals,  the  seeds,  which  are 
protected  by  an  indigestible  covering,  are  often  swallowed 
and  voided  at  a  distance.  The  attractive  succulent 
part  of  the  fruit  may  be  the  inner  layers  of  the  pericarp 
(melon,  raspberry,  date,  etc.)  or  special  ingrowths  from 
the  pericarp  (orange),  or  it  may  be  the  perianth  leaves 
(mulberry),  or  the  receptacle  of  the  flower  which  has 
become  fleshy  in  the  fruit  (strawberry,  apple),  or  of 
the  whole  inflorescence.  The  seeds  of  such  fruits  are 
nearly  always  protected  by  a  layer  of  hard-walled  cells, 
which  resist  the  digestive  enzymes  secreted  in  the 
alimentary  canal  of  the  animal  and  thus  prevent 
the  substance  of  the  seed  from  being  digested.  In 
most  cases  this  is  the  testa  of  the  seed,  but  when  the 
succulent  part  of  the  fruit  is  the  receptacle  or  perianth, 
it  is  the  whole  pericarp  (strawberry,  apple,  fig,  mulberry), 
or  where  the  outer  layers  of  the  pericarp  alone  are 
succulent,  it  is  the  inner  layer,  i.e.  the  "  stone  "  of 
"  stone  fruits,"  the  seed  itself  (the  kernel)  being  digestible 
once  the  stone  is  removed. 

Very  often  the  seeds  of  succulent  fruits  stick  to  the 
beaks  of  birds  pecking  at  the  fruit,  because  they  are 
surrounded  by  a  sticky  pulp,  and  are  later  rubbed  off 
or  fall  off  when  the  pulp  dries.  The  seeds  of  mistletoe 
are  distributed  in  this  way.  The  birds  peck  at  the 
mistletoe  berries  and  the  seeds  stick  to  their  bills. 
They  clean  their  bills  by  rubbing  them  on  the  bark  of 
trees,  and  the  seeds  catch  in  the  crevices,  where  they 


372  THE   FRUIT 

germinate.  The  mistletoe  is  a  parasite  on  trees,  and 
the  modified  roots  (haustoria)  produced  by  the  seedling 
penetrate  the  bark  and  into  the  wood  of  the  tree. 

(3)  Dispersal  by  Water  Carriage.— Many  seeds  and 
fruits  float  when  they  drop  into  water,  and  may  thus 
be  carried  considerable  distances  by  rivers,  germinating 
if  they  are  stranded  in  a  suitable  spot.  Many  seeds 
become  waterlogged  and  sink  after  a  short  time,  but 
others  float  for  long  periods,  or  indefinitely.  The  seeds 
of  many  riverside  plants  are  surrounded  by  a  tissue 
containing  much  air,  and  are  thus  specially  equipped 
for  floating. 

The  seeds  and  fruits  of  a  large  number  of  species  of 
tropical  seashore  plants — both  trees  and  herbs — have 
such  a  floating  tissue  and  are  able  to  stand  long  soaking 
in  salt  water  without  harm.  They  are  constantly 
carried  about  in  shore  currents  and  stranded  on  the 
coast,  where  they  germinate  and  produce  fresh  vegeta- 
tion. Newly  formed  islands  of  coral  or  volcanic  material 
are  largely  populated  by  plants  in  this  way,  and  this 
feature  of  tropical  coast  plants  is  also  largely  responsible 
for  the  great  uniformity  of  this  vegetation  over  very 
wide  areas.  The  coconut  is  the  best-known  example, 
the  very  thick  woody  pericarp  being  very  light  in 
proportion  to  its  bulk  and  causing  the  fruit  to  float 
indefinitely. 

It  has  been  shown  that  this  character  of  coastal 
plants  has  been  developed  in  different  species  from 
other  causes  not  related  to  the  capacity  for  floating. 
But  it  is  clear  that  the  species  whose  seeds  and  fruits 
can  float  will  have  a  much  greater  chance  of  successful 
propagation,  and  especially  of  wide  dispersal,  than  if 
they  had  seeds  or  fruits  which  sank  at  once  or  could 
not  long  resist  soaking  in  salt  water.  There  is  thus  a 


PRACTICAL   WORK  373 

strong  selective  effect  which  will  inevitably  lead  to  the 
prevalence  of  these  species  in  this  particular  habitat. 
The  same  general  consideration  holds  of  most  of 
these  so-called  "  adaptive  "  characters.  They  have  not 
in  the  first  instance  appeared  in  relation  to  the  conditions 
in  which  they  are  of  special  use  to  the  organism,  but 
the  existence  of  the  character  may  favour  the  spread 
of  the  species  when  these  conditions  arise,  and  may 
even  in  extreme  cases  secure  its  continued  existence 
in  circumstances  which  would  lead  to  the  extinction 
of  a  species  not  so  equipped. 

PRACTICAL  WORK. 

Development  of  Seed  and  Embryo. 

(1)  Slit  open  the  fresh  ovaries  of  Shepherd's  Purse  (Capsella), 
choosing  those  from  which  the  petals  have  just  fallen,  and  older 
stages  up  to  the  full-sized  capsules.     Draw  under  the  low  power 
seeds  of  different  stages  of  development. 

Squeeze  the  ovules  of  different  ages  under  a  coverslip  so  as  to 
press  out  the  embryos,  and  draw  various  stages  in  the  develop- 
ment of  the  embryo.  Note  the  suspensor  and  the  spherical 
embryo  in  the  young  stage,  and  the  subsequent  development 
of  the  two  cotyledons. 

Dehiscent  Fruits. 

(2)  Examine  the   young   fruit   of  the   Sweet   Pea    (Lathyrus) 
projecting   from   the   remains   of   the   flower.     Split   the   ovary 
down  and  note  the  two  rows  of  young  seeds  attached  to  the 
upper  edge.     Note  the  similarity  of  the  single  carpel  here  with 
the  carpel  of  Caltha  (p.  359).     In  the  ripe  fruit  note  the  mature 
seeds  and  the  mode  of  dehiscence. 

(3)  Examine    any    available    examples    of    syncarpous,    dry 
dehiscent  fruits  (capsules). 

Indehiscent  Dry  One-Seeded  Fruits. 

(4)  Cut  the  young  "  aggregate  fruits  "  of  Herb-bennet  (Geum) 
longitudinally  in  half  so  as  to  show  the  insertion  of  the  individual 
achenes  on  the  receptacle.     Note  that  a  hook  is  developed  in 
the  style  just  below  the  stigma,  which  falls  off. 


374  THE   FRUIT 

(5)  Draw  a  single   fruit   of   Dandelion  (Taraxacum),  showing 
the  stalked  pappus  surmounting  the  one-seeded,  dry  indehiscent 
ovary. 

(6)  Draw  the  fruit  of  sycamore  (Acer],  consisting  of  two  dry 
one-seeded  carpels.     The  pericarp  of  each  has  grown  out  in  the 
flower  to  form  a  long  curved  wing.     Note  that  the  two  carpels 
easily  separate.     Dissect  one  of  the  seeds,  and  note  the  thin 
testa  and  the  green  coiled  embryo  with  long  strap-shaped  cotyledons. 

Examine  the  flower  of  the  sycamore  and  note  the  developing 
wings  of  the  ovary,  and  that  each  carpel  originally  contained  two 
ovules. 

Succulent  Fruits. 

(7)  In  the  young  cherry  (Prunus)  note  the  suture  (joined  margins) 
of  the  single  carpel,  the  scar  where  the  style  has  fallen  off,  and  the 
flattened  disc-like  (once  basin-shaped)  receptacle.     Cut  a  longi- 
tudinal section,  and  distinguish  the  three  layers  of  the  pericarp 
(skin,  flesh  and  stone).     In  the  young  seed  distinguish  the  testa 
nucellus  and  gelatinous  endosperm.     Look  for  the  young  embryo 
at  one  end. 

Compare  the  ripe  cherry.  Break  the  stone  and  dissect  the 
seed  (kernel).  The  embryo  with  two  cotyledons  now  occupies 
the  whole  space. 

(8)  Draw  a  median  longitudinal  section  of  an  Apple  or  Pear 
flower  (Pyrus)  from  which  the  petals  have  fallen  and  in  which 
the  receptacle  is  distinctly  swollen.     Note  the  tubular  receptacle, 
with  the  sepals  attached  to  its  rim,  and  the  remains  of  the  stamens 
just  inside  it.     The  ovaries  of  the  carpels  are  buried  at  the  base 
of  the  tube,  and  the  styles  pass  up  through  the  centre. 

(9)  Draw  transverse   and   longitudinal  sections   of   a  partly 
grown  apple  or  pear  (about  three-quarters  of  an  inch  in  diameter) 
showing  the  relation  of  the  carpels  (core)  to  the  fleshy  receptacle. 

(10)  Draw  similar  sections  of  a  ripe  apple  or  pear,  identifying 
the  structures  visible  with  those  seen  in  the  earlier  stages.     Note 
the  withered  styles  still  lying  in  the  central  tube,  the  remains 
of  the  stamen  still  often  present  within  the  sepals  :   the  vascular 
cylinder  running  from  the  top  of  the  stalk  to  the  base  of  the  carpels 
and  its  continuation  through  the  central  column  serving  the 
placentae  ;    also  the  bundles  running  through  the  flesh  of  the 
receptacle. 


CHAPTER    XXIII 

THE  SEED   AND   ITS   GERMINATION 

WE  have  already  seen  (p.  363)  that  different  seeds  vary 
very  much  in  the  size  attained  by  the  embryo  when  the 
seed  is  ripe,  apart  from  the  dimensions  of  the  seed 
itself,  which  varies  from  the  microscopic  seeds  of  orchids 
to  the  enormous  seed  of  the  coconut.  During  develop- 
ment the  embryo  absorbs  more  or  less  of  the  endosperm, 
but  in  some  cases  it  is  still  surrounded  by  endosperm 
in  the  ripe  seed,  while  in  others  it  absorbs  the  whole 
of  the  endosperm  and  comes  to  occupy  the  entire  space 
within  the  testa.  Seeds  in  which  endosperm  is  still 
present  at  maturity  are  called  endospermic  seeds,1 
and  those  which  have  no  endosperm  left  when  they 
are  ripe  are  non-endospermic.*  Examples  of  the  former 
are  wheat,  maize  (and  the  cereals  generally),  castor 
oil,  date,  etc.  :  of  the  latter,  bean,  pea,  acorn,  etc.  In 
non-endospermic  seeds  the  embryo,  and  especially  the 
cotyledons,  are  often  large  and  swollen,  their  cells  packed 
with  the  food  material  which  in  the  endospermic  seeds 
would  remain  in  the  endosperm  till  the  time  of  germina- 
tion. This  food  material  takes  the  forms  with  which 
we  have  already  become  familiar.  The  nitrogenous 
organic  substance  is  in  the  form  of  solid  grains  of  protein 
material,  the  non-nitrogenous,  either  as  carbohydrate — 
starch  grains,  or  more  rarely  as  thick  cellulose  walls 

1  The  old  name  is  "  albuminous  seeds,"  from  the  analogy  of  the 
"  white  "  or  "  albumen  "  of  a  bird's  egg,  which,  like  the  endosperm, 
is  a  store  of  food  for  the  developing  embryo. 

*  Or  "  exalbuminous." 


376  THE   SEED  AND  ITS   GERMINATION 

e.g.  date) — or  as  fats,  in  the  form  of  oil  drops,  plant  fats 
being  liquid  at  ordinary  temperatures  :  the  cytoplasm 
of  the  cells  of  the  endosperm  (and  perisperm  if  present) 
or  of  the  embryo  itself  being  densely  packed  with  these 
substances.  The  proportion  of  water  in  a  seed  is  very 
much  less  than  in  the  actively  growing  vegetative  parts 
of  the  plant. 

Conditions  of  Germination. — So  long  as  the  ripe  seed 
is  kept  dry  the  protoplasm  of  its  cells  remains  in  a 
dormant  condition.  Seeds  may  remain  alive  in  this 
condition  for  many  years,  and  though  the  germination 
of  "  inummy  wheat  "  has  not  been  verified,  such  an 
occurrence  is  not  by  any  means  out  of  the  question. 
Seeds  have  been  known  to  germinate  after  keeping 
dry  for  a  century,  though  some  seeds  die  within  a 
comparatively  short  time. 

The  first  requisite  for  germination  is  liquid  water 
Some  ripe  seeds  (e.g.  acorns)  contain  enough  water  to 
germinate  at  once  if  evaporation  is  checked  by  keeping 
them  in  saturated  air,  but  once  they  have  lost  a  certain 
proportion  of  their  water  they  must  have  more  supplied 
to  them  from  outside  in  order  to  germinate.  The 
second  requisite  is  free  oxygen.  The  living  cells  of  the 
embryo  must  be  able  to  respire  in  order  to  liberate 
the  energy  necessary  for  growth ;  and  both  water  and 
oxygen  are  necessary  to  the  chemical  changes  which 
precede  and  accompany  germination.  The  third  is 
a  suitable  temperature.  At  or  below  o°  C.  protoplasm  is 
inactive,  and  germination  cannot  occur  any  more  than 
can  the  growth  processes  of  an  ordinary  growing  plant. 
Above  this  temperature  the  chemical  changes  on  which 
germination  depends  takes  place  more  and  more  quickly, 
so  that  the  higher  the  temperature,  up  to  a  certain  point, 
the  more  rapid  germination  will  be.  But  above  a 


PROCESSES   PREPARATORY   TO   GERMINATION        377 

certain  temperature  the  working  of  the  protoplasmic 
"  machine  "  is  thrown  out  of  gear  and  death  results. 

Enzyme  Formation  and  Activity. — One  of  the  first 
processes  preparatory  to  germination  is  the  production 
of  enzymes  in  the  cells  of  the  endosperm  and  embryo, 
by  the  activity  of  which  the  various  reserve  foods  are 
converted  into  soluble  forms.  The  starch  is  acted 
upon  by  diastase,  the  cellulose  (as  in  the  date  seed) 
by  cytase,  the  fats  by  the  Upases,  the  proteins  by 
various  proteases  (see  p.  45).  The  carbohydrates  and 
fats  thus  give  rise  to  great  quantities  of  sugars,  and 
these  are  largely  used  for  respiration,  i.e.  for  the  libera- 
tion of  energy  which  is  seen  in  the  active  growth  of  the 
embryo  :  part  of  this  energy  takes  the  form  of  heat. 
Thus  the  main  interchanges  between  the  germinating 
seed  and  its  surroundings  are  first  of  all  the  active 
absorption  of  water  resulting  in  the  swelling  of  the  seed 
(cf.  the  experiment  with  peas,  p.  60),  then  the  absorption 
of  free  oxygen,  and  the  evolution  of  large  quantities 
of  carbon  dioxide  and  heat  (p.  89).  Some  of  the  sugar 
is  used  for  forming  new  cell  walls,  while  the  soluble 
nitrogenous  substances  into  which  the  solid  proteins 
are  converted  by  the  activity  of  proteolytic  enzymes 
are  worked  up  again  to  form  the  basis  of  the  new 
protoplasm  produced  in  active  cell  division. 

Structural  Changes  in  Germination.  Epigeal  and 
Hypogeal  Cotyledons. — Most  of  the  cells  of  the  embryo, 
especially  if  it  is  small  and  relatively  slightly  developed 
in  the  resting  stage,  actively  divide  during  the  first 
stage  of  growth,  but  cell  division  soon  becomes  mainly 
concentrated  in  the  apical  meristems.  The  actual 
growth  of  the  embryo  into  the  seedling  takes  place 
very  largely  by  vacuolation  and  consequent  increase 
in  size  of  the  cells. 


37§  THE   SEED  AND  ITS  GERMINATION 

The  first  external  change  is  the  pushing  out  of  the 
radicle  into  the  soil,  sometimes  through  the  micropyle, 
sometimes  through  a  break  in  the  testa,  into  the  soil, 
where  it  bends  straight  downwards  to  become  the  tap- 
root, and  often  grows  to  a  considerable  length  before 
the  shoot  appears.  In  plants  with  a  "  fibrous  "  root 
system  the  development  of  the  taproot  soon  stops,  and 
other  roots  grow  out  from  the  base. 

In  the  development  of  the  shoot  we  have  to  distinguish 
two  types  of  germination.  In  the  first,  by  far  the  com- 
monest type,  the  hypocotyl  increases  enormously  in 
length,  thrusting,  or  sometimes  pulling,  the  cotyledons, 
with  the  epicotyledonary  bud  lying  between  them,  out 
of  the  testa  (Fig.  63,  A).  The  cotyledons  are  said  to 
be  epigeal «  (raised  above  the  soil).  The  cotyledons  are 
often  held  with  a  certain  firmness  by  the  testa,  and 
the  base  of  the  hypocotyl  being  fixed  by  the  primary 
root  in  the  soil,  the  hypocotyl  curves  in  elongating, 
setting  up  a  strain  and  eventually  hauling  the  cotyledons 
out  of  the  testa.  As  soon  as  these  are  free  the  hypocotyl 
straightens  out,  carrying  the  cotyledons  on  its  summit, 
and  these  expand  and  turn  green  (if  they  are  not  green 
already),  standing  out  on  each  side  in  a  horizontal 
or  upwardly  inclined  position. 

This  is  the  end  of  the  first  stage  of  growth  of  the 
seedling,  which  thus  becomes  an  autonomous  green 
plant,  no  longer  dependent  on  the  food  in  the  endosperm, 
which  has  been  mostly  absorbed  by  the  time  the 
hypocotyl  elongates.  Many  epigeal  cotyledons  are 
thin,  like  the  later  formed  foliage  leaves,  but  in  some 
cases  the  fleshy  cotyledons  of  non-endospermic  seeds 
are  raised  above  the  ground  (French  bean). 

Sometimes  the  cotyledons  are  not  at  once  successfully 

1  Greek  eiri,  on,  and  yrj,  earth. 


TYPES   OF   GERMINATION 


379 


freed  from  the  testa,  which  is  carried  up,  sticking  on  to 
one  or  both.  In  most  cases  it  is  soon  shaken  off  by 
their  further  growth,  but  occasionally  seedlings  are 


FIG.  63. — Types  of  germination.  A,  endospermic  seed  with  epigeal 
cotyledons.  B,  non-endospermic  seed  with  hypogeal  cotyledons. 
C,  endospermic  seed  with  hypogeal  cotyledons,  which  remains 
embedded  in  the  endosperm  and  acts  as  a  sucker  ;  end.,  endosperm ; 
cot.,  cotyledon  ;  ep.,  epicotyl ;  rad.,  radicle. 

handicapped,  or  even  killed  from  this  cause.  In  the 
marrow  a  projection  of  tissue  is  developed  from  the 
base  of  the  hypocotyl  which  holds  down  one  side  of 
the  testa  (the  seed  being  here  flattened  and  the  testa 


380  THE   SEED  AND   ITS   GERMINATION 

consisting  of  two  valves  which  grip  the  cotyledons 
rather  like  the  two  shells  of  an  oyster)  and  acts  as  a 
fulcrum  on  which  the  elongating  hypocotyl  works  in 
pulling  the  cotyledons  out  of  the  testa. 

In  the  second  type  of  germination  the  cotyledons 
remain  within  the  seed  (hypogeal1).  The  hypocotyl 
does  not  elongate,  but  the  epicotyledonary  bud  at 
once  grows  out,  at  the  expense  of  the  food  stored  in 
the  cotyledons,  when  these  are  large  and  swollen  as 
in  non-endospermic  seeds  (Fig.  63,  B),  (bean,  acorn)  ; 
while  in  endospermic  seeds  the  cotyledon  acts  as  a 
"  sucker,"  remaining  embedded  in  the  endosperm, 
absorbing  and  transmitting  the  soluble  foodstuff  from 
the  endosperm  to  the  growing  parts  of  the  seedling 
(Fig.  63,  C).  This  is  well  seen  in  the  monocotyledonous 
date  seed  in  which  the  peg-like  cotyledon  embedded  in 
the  endosperm  produces  cytase,  which  attacks  the  solid 
cellulose  of  the  endosperm  and  absorbs  the  sugar  pro- 
duced. In  the  cereals  a  special  organ,  the  scutellum, 
lies  between  the  axis  of  the  embryo  and  the  endosperm, 
and  its  surface  cells  secrete  diastase  which  passes  into 
the  starchy  endosperm,  attacks  and  hydrolyses  the 
starch  grains,  the  sugar  produced  being  absorbed  by 
the  scutellum.  In  all  cases  the  endosperm  or  cotyledon 
cells  contain  protein  grains,  the  nitrogenous  material 
represented  by  which  is  of  vital  importance  in  the  forma- 
tion of  new  protoplasm,  though  much  less  in  amount 
than  the  carbohydrate  material.  Much  of  the  sugar 
derived  from  this  latter  is  broken  up  in  the  intense 
respiration  which  occurs  in  the  germinating  seed. 

The  plant  may  in  fact  be  regarded  as  living  at  its 
highest  intensity  during  germination.  At  no  other  time 
is  there  so  extensive  and  rapid  a  mobilisation  of  food 

1  VTTO  and  yfj,  below  the  earth. 


USE   OF   SEEDS   BY   MAN  381 

reserves  for  purposes  of  growth,  whether  on  the  side 
of  the  formation  of  new  protoplasm  and  cell  walls,  or 
on  the  side  of  the  expenditure  of  energy.  A  comparable 
situation  occurs  at  the  resumption  of  growth  in  a  perennial 
plant  after  the  winter  rest,  when  the  winter  buds 
on  the  rhizome  or  woody  stem  grow  out  to  form  new 
shoots ;  but  it  is  not  so  intense  or  concentrated  as  in 
the  seedling. 

Use  of  Seeds  by  Man.— Owing  to  the  concentrated 
supplies  of  organic  food  they  contain  seeds  form  by 
far  the  most  important  vegetable  foodstuffs  of  man. 
The  great  bulk  of  his  cultivated  crops  are  grown  for 
the  seed.  First  in  importance  among  these  come  the 
cereals  :  wheat,  rice,  maize,  millet,  barley,  oats  and  rye, 
which  all  belong  to  the  great  family  of  grasses  (Gramineae). 
These  are  all  starchy  seeds,  but  with  a  considerable 
proportion  of  protein.  The  protein  varies  in  different 
cereals,  for  instance  wheat,  the  standard  food  of  the 
white  man,  contains  considerably  more  than  rice,  the 
standard  food  of  the  natives  of  large  parts  of  the  tropics. 
Barley  is  grown  in  central  and  north  Europe  mainly 
for  malting,  the  seeds  being  germinated  (malt)  and  the 
reserves  rendered  soluble,  so  that  they  can  be  dissolved 
out  in  hot  water  to  form  the  basis  (wort)  for  the  making 
of  beer  (p.  132).  But  in  other  parts  of  the  world 
barley  is  grown  for  bread-making. 

Next  in  importance  to  the  cereals  come  the  pulses 
(beans,  peas,  lentils,  etc.),  which  belong  to  the  legume 
family,  the  fruit  consisting  of  a  single  dehiscent  carpel 
(p.  368).  Most  of  these  are  also  rich  in  starch,  but 
contain  a  particularly  high  proportion  of  proteins. 
The  pulses  are  also  widely  used  as  foodstuffs,  both  for 
men  and  cattle,  but  not  so  extensively  as  the  cereals. 

Finally,  there    are    the    fat    (oil)    containing    seeds. 


382  THE  SEED  AND  ITS  GERMINATION 

Of  these  certain  species  of  palm  are  among  the  chief. 
The  West  African  oil-palm  (Elceis  guineensis)  and 
the  coconut  palm  (Cocos  nucifera)  are  the  two  leading 
species.  The  former  is  a  constituent  of  most  of  the 
native  food  in  West  Africa,  and  it  has  been  extensively 
planted  in  other  parts  of  the  continent.  In  comparatively 
recent  years  a  great  export  trade  in  "  palm  kernels  "  has 
been  developed,  and  from  these  the  oil  is  extracted 
for  various  purposes,  prominent  among  which  is  the 
manufacture  of  margarine.  The  coconut  is  used  very 
extensively  by  the  natives  throughout  the  damp  tropics 
for  food,  lighting  and  other  things,  and  the  oily  endosperm 
is  very  extensively  sun-dried  and  exported  as  copra, 
which  is  used  for  margarine,  soap-making,  etc. 

Other  important  oil-bearing  seeds  are  the  "  soy  bean  " 
(a  leguminous  plant),  linseed  (the  seed  of  the  flax), 
and  cottonseed  (the  seed  of  the  cotton  plant),  the  oils  of 
which  are  all  used  in  making  food  and  for  a  variety 
of  industrial  purposes.  After  most  of  the  oil  is 
extracted  the  residue  is  utilised  as  cattle  food  ("  cake  "). 
Relation  of  the  Size  of  the  Seed  to  Dispersal  and  to 
the  Chances  of  Life  of  the  Seedling. — A  large  seed  con- 
taining a  large  amount  of  food  material  naturally  enables 
a  large  seedling  to  be  produced  in  a  comparatively  short 
time,  just  as  a  considerable  shoot  system  can  be  quickly 
produced  from  a  bulb  or  tuber  :  whereas  a  seedling 
produced  from  a  small  seed  uses  up  its  reserves  very 
quickly,  and  has  the  laborious  work  of  slowly  building 
up  new  organic  substance  from  inorganic,  with  a  root 
and  leaf  surface  which  is  at  first  of  very  limited  extent. 
Provided  it  finds  a  suitable  spot  for  germination  and 
growth,  the  seedling  derived  from  a  large  seed  thus 
has  a  great  initial  advantage  in  competing  with  other 
plants  for  space  and  light,  because  it  is  less  likely  to  be 


DEATH  RATE  OF  SEEDLINGS  383 

smothered  and  to  have  its  light  cut  off  by  the  vege- 
tation around  it 

Plants  producing  small  seeds,  on  the  other  hand, 
have  two  great  advantages.  The  seeds  can  be  produced 
in  far  greater  numbers  with  an  equal  supply  of  food 
from  the  parent  plant,  and  they  are  much  more  readily 
dispersed,  so  that  there  is  more  chance  of  many  of 
them  reaching  comparatively  distant  spots  where  they 
can  germinate  and  establish  themselves,  and  thus 
the  species  has  a  better  chance  of  wide  distribution 
and  ultimate  survival.  The  two  most  widely  distributed 
and  numerous  families  of  flowering  plants,  the  grasses 
(Gramineae)  and  the  composites  (Compositae),  both  have 
small  one-seeded  fruits,  and  distribution  is  often  facili- 
tated in  the  latter  case  by  the  pappus  (p.  370). 

Death  Rate  and  Competition. — The  death  rate  of 
seeds  and  seedlings  in  nature,  like  that  of  all  young 
organisms,  is  enormous.  Besides  the  large  number  of 
seeds  which  fall  in  places  where  they  cannot  germinate 
and  the  large  number  that  are  eaten  by  animals,  many 
seeds  which  do  begin  to  germinate  are  killed  at  an  early 
stage  by  finding  no  suitable  soil  in  which  they  can  root, 
by  being  smothered  or  cut  off  from  light  by  other  plants, 
or  by  the  attacks  of  fungi  or  small  insects.  At  a  rather 
later  stage  very  many  are  eaten  off  by  rodents  or  by 
browsing  animals.  No  seedling  of  a  woody  plant  can 
survive,  for  instance,  in  heavily  pastured  grassland. 
Perennial  herbaceous  plants  like  the  grasses  survive 
in  such  land  because  of  their  underground  and  surface 
shoot  systems,  which  possess  buds  that  grow  out  as  soon 
as  the  upper  shoots  are  eaten  off. 

If  we  suppose  an  annual  plant  to  produce  only  ten 
seedlings  a  year,  and  all  of  these  survive  and  them- 
selves produce  seed,  we  should  have  in  the  twelfth 


384  THE   SEED  AND   ITS   GERMINATION 

year  a  million  million  individuals  of  that  species,  the 
offspring,  in  the  twelfth  generation,  of  a  single  plant. 
Many  species  at  the  present  time  are  actively  spreading 
and  increasing  their  territory,  though  the  increase  is 
never  more  than  a  minute  fraction  of  that  which  would 
occur  if  all  offspring  survived.  Many  others  are  decreas- 
ing, not  holding  their  own  in  the  struggle  for  space, 
light  and  water.  Probably  a  much  larger  number  of 
species  are  more  or  less  holding  their  own.  It  is  by 
competition  between  individuals  of  the  same  and  different 
species  that  this  approximate  balance  is  maintained. 

Significance  of  the  Seed  in  the  Evolution  of  the  Plant 
World. — The  ovule,  and  the  seed  into  which  the  ovule 
develops,  represent  the  culmination  of  the  adaptation 
of  the  sexual  reproductive  processes  to  the  conditions 
of  plant  life  on  land,  where  an  external  supply  of  water 
on  the  surface  of  the  soil  is  no  longer  available.  We 
saw  (in  Chapter  XIV)  the  preparatory  stages  which 
necessarily  preceded  the  appearance  of  seeds,  and  which 
developed  many  millions  of  years  ago,  during  the  Palaeo- 
zoic age,  in  the  ancestors  of  our  modern  seed  plants. 
These  preparatory  stages  are  still  represented  in  the 
few  existing  heterosporous  Pteridophytes.  Instead  of 
uniform  spores  which  germinate  to  form  green  free- 
living  plants — the  prothalli  of  such  plants  as  ferns — 
which  produce  the  sexual  organs,  the  heterosporous 
forms  produce  two  kinds  of  spores,  the  large  megaspores 
and  the  small  rnicrospores,  which  germinate  to  form 
microscopic  prothalli,  dependent  on  the  spores,  and 
respectively  producing  the  male  and  the  female  gametes. 
The  process  of  conjugation  still  depends,  however, 
on  external  liquid  water  in  which  the  flagellate 
male  gametes  can  swim,  and  this  dependence  severely 
restricts  the  habitats  in  which  such  plants  can  live. 


SIGNIFICANCE   OF   THE    SEED   IN   EVOLUTION        385 

The  progress  to  the  development  of  the  ovule  depends 
on  the  retention  of  the  megaspore  within  the  megasporan- 
gium  and  the  appearance  of  some  means  other  than 
their  own  locomotion  in  water  of  bringing  the  male 
gametes  to  the  female.  We  do  not  know  all  the  steps 
in  the  evolutionary  process,  because  we  have  not 
discovered  the  fossil  forms  which  were  the  actual 
ancestors  of  our  modern  seed  plants.  But  the  essential 
new  features  are  two,  first  the  appearance  of  a  wall  of 
cells  to  the  megasporangium  (nucellus  of  the  ovule)  and 
of  one  or  two  coats  of  cells  covering  the  nucellus,  and 
secondly  of  the  germination  of  the  microspore  (pollen 
grain)  to  form  a  germ  tube  (pollen  tube)  which  can 
grow  to  the  neighbourhood  of  the  egg,  carrying  the  male 
gametes  with  it.  A  further  feature  is  the  closure  of  the 
leaf  (carpel)  round  the  megasporangia  (ovules).  This  is 
not  found  in  the  most  primitive  existing  seed  plants,  such 
as  the  pines  and  firs,  which  are  called  "  Gymnosperms,"  l 
because  their  ovules  are  not  enclosed  in  carpels. 

The  pollen  grains  are  brought  to  the  ovule  in  Gym- 
nosperms, and  to  a  special  receptive  organ  of  the  carpel 
(stigma)  in  most  flowering  plants,  by  the  wind  or  by 
insects,  and  the  perfection  of  the  method  of  their 
transport  has  undoubtedly  increased  the  chances  of 
fertilisation  in  a  great  variety  of  species.  From  the 
stigma  to  the  ovule  the  growth  of  the  pollen  tube  and 
the  life  of  the  male  gametes  it  encloses  are  protected  by 
the  fact  that  it  takes  place  inside  the  tissue  of  the  carpel. 
The  female  gamete  is  well  protected  by  nucellus,  ovule 
coats  and  carpel  wall.  All  these  arrangements  enable 
the  process  of  fertilisation  to  be  carried  out  in  flowers 
exposed  to  dry  air  in  which  exposed  gametes  would 
at  once  be  killed. 


Greek  -yv^vog,  naked,  and  GTiipna.,  seed. 
25 


386  THE   SEED   AND   ITS   GERMINATION 

But  the  withdrawal  of  the  process  of  fertilisation  to 
the  interior  of  the  well-protected  ovule  has  another  great 
advantage- — it  enables  the  zygote,  the  result  of  fertilisa- 
tion, to  be  developed  into  an  embryo  in  a  protected 
position,  and  to  be  supplied  by  the  parent  plant  with 
the  food  necessary  to  its  development  in  that  position. 
In  this  way  the  young  plant  is  able  to  establish  itself 
in  a  short  time  after  the  seed  germinates — it  has  a 
chance  of  penetrating  quickly  into  the  damp  soil  below 
the  surface  because  it  has  a  store  of  food  at  its  disposal 
for  rapid  growth. 

In  the  fern  plant  the  sexual  generation  is  a  free- 
living  green  plant  produced  by  the  germination  of  the 
spore  on  the  soil.  The  young  sporophyte  is  "  parasitic  " 
on  the  prothallus  during  the  first  stage  of  its  growth. 
In  the  seed  plant  the  prothallus  is  reduced  to  its  lowest 
terms  and  is  "  parasitic  "  on,  or  rather  in,  the  mother 
sporophyte  (the  "  vegetative  "  plant).  But  not  only  so  : 
the  new  sporophyte  produced  from  the  zygote  is  also 
parasitic  on  (i.e.  it  derives  its  food  from)  the  mother 
sporophyte,  and  is  protected  by  it  in  the  altered 
megasporagium  (seed)  up  to  a  certain  stage  of  its 
development,  when  it  can,  on  the  germination  of  the 
seed,  rapidly  become  free  living. 

Thus  the  seed  plant  has  successfully  solved  the  problem 
of  sexual  reproduction,  and  of  giving  the  new  generation 
a  good  start  in  life  under  conditions  which  the  lower 
plants  are  quite  unable  to  meet.  The  principle  of  the 
mating  of  the  delicate  and  easily  destroyed  gametes 
inside  the  body  of  the  parent,  and  of  the  protection  of 
the  embryo  during  the  early  stages  of  its  life,  is  the 
same  that  we  see  successful  in  the  higher  animals, 
though  the  details  of  the  mechanism  are  so  widely 
different.  Taken  together  with  the  power  of  maintain- 


PRACTICAL   WORK  387 

ing  and  increasing  the  aerial  shoot  from  year  to  year, 
thus  permitting  of  that  great  increase  in  the  bulk  of 
the  body  which  has  given  us  the  trees,  the  dominant 
plants  over  a  large  part  of  the  earth's  surface,  the 
production  of  ovules  and  seeds  has  enabled  the  higher 
plants  almost  to  cover  the  face  of  the  globe. 

PRACTICAL  WORK. 

Seeds  and  Seedlings. 

A.  THE  BROAD  BEAN   ( Vicia  Faba)  :    a  seed  without  endosperm 

and  with  hypogeal  cotyledons. 

(1)  Make  drawings  of  the  side  and  front  views  of  the  seed 
showing  the  long  scar  of  attachment  to  the  pod  and  the  position 
of  the  micropyle  at  one  end  of  the  scar.     [By  squeezing  a  bean 
which  has  been  thoroughly  soaked  a  drop  of  water  can  be  forced 
out  through  the  micropyle.]     Pull  the  testa  from  a  soaked  bean 
to  show  the  embryo,  which  completely  fills  the  seed.     Detach 
carefully  one  of  the  massive  cotyledons,  and  draw  the  internal 
surface  of  the  other  with  the  curved  epicotyl  and  the  straight 
radicle  at  one  side. 

(2)  Sketch  a  series  of  germinating  seeds  and  seedling  in  different 
stages,    marking   cotyledons,    epicotyl,   foliage   leaves    (the   lowest 
are   rudimentary),    axillary    buds,    terminal    bud,   primary   root, 
lateral  roots.     The  hypocotyl  remains  short. 

B.  THE  VEGETABLE  MARROW  (Cucurbita)  :  a  seed  with  perisperm 

and  thick  epigeal  cotyledons. 

(3)  Note  the  flatness  of  the  seed,  the  scar  of  attachment  (pit- 
like)   and   the   adjacent  micropyle.      Split   the   seed  lengthwise 
and  observe  the  greenish  layer  of  perisperm  (nucellus),  two  rather 
thick  cotyledons  (though  much  thinner  than  in  the  bean),  with 
very  minute  epicotyledonary  bud  between,  and  radicle. 

(4)  Make   a   series   of   sketches   from   stages   illustrating   the 
elongation  of  the  hypocotyl,  the  pulling  of  the  cotyledons  from 
the  testa,  the  straightening  of  the  hypocotyl  and  the  expansion 
and  greening  of  the  cotyledons.     Mark  the  various  organs  of 
the  seedling,  the  epicotyledonary  leaves  differing  in  shape  from 
the  cotyledons  and  the  "  peg  "  or  "  heel  "  at  the  base  of  the 
hypocotyl,  which  holds  down  one  valve  of  the  testa  while  the 
cotyledons  are  extracted. 


388  THE  SEED  AND   ITS   GERMINATION 

C.  THE  CASTOR-OIL  PLANT  (Ricinus)  :  A  seed  with  oily  endosperm 
and  epigeal  cotyledons. 

(5)  Remove  the  mottled  brittle  testa  (with  its  wart-like  swelling 
at  one  end)  from  the  seed,  exposing  the  white  mass  of  oily  endo- 
sperm :  split  this  open  very  carefully  and  observe  the  embryo 
in    the    middle.     The    very    thin    membranous    cotyledons    are 
extremely  delicate   and   easily   tear.     They  extend   almost  the 
whole  length  of  the  seed  and  have  a  complete  system  of  veins 
already  developed.    Draw  the  embryo,  noting  the  short  cylindrical 
hypocotyl,  continuous  with  the  radicle,  and  the  epicotyledonary 
bud. 

(6)  Sketch  a  series  of  stages  in  the  development  of  the  seedling, 
showing  the  gradual  straightening  and  elongation  of  the  hypocotyl 
(which  may  reach  a  length  of  8  to  12  inches  and  a  considerable  thick- 
ness), the  large  flat  cotyledons  and  the  development  of  the  epicotyl- 
edonary shoot.     Note  the  buds  in  the  axils  of  the  cotyledons  and 
the  well-developed  root  system. 


(7)  If  available,  compare  maize,  wheat  and  date  seedlings, 
showing  endosperm  and  "sucking"  organs  :  also  the  very  small 
seedlings  with  epigeal  cotyledons  of  some  of  our  common  weeds 
and  garden  crops. 


CHAPTER  XXIV 
CONCLUSION 

IN  the  foregoing  chapters  we  have  become  acquainted 
with  some  facts  about  living  beings,  especially  plants, 
and  we  may  now  briefly  consider  the  chief  lessons 
about  life  that  we  learn  from  them. 

In  the  earlier  chapters  we  saw  that  life  everywhere 
depends  upon  a  substance  we  call  protoplasm — that 
this  is  present  wherever  we  find  life — in  all  organisms 
from  amoeba  to  man,  and  from  bacteria  to  the  seed 
plants — and  that  it  is  not  present  in  lifeless  objects. 
This  protoplasm,  as  we  saw,  has  a  more  or  less  definite 
chemical  constitution.  It  is  not  itself  a  chemical 
compound,  but  a  mixture  made  up  in  all  organisms  of 
the  same  classes  of  compounds.  In  physical  structure, 
too,  it  is  everywhere  similar,  belonging  to  a  kind  of 
substance — or  rather  a  special  condition  of  matter 
known  as  the  colloid  condition — which  consists  of  very 
minute  particles  or  droplets  of  one  substance  (disperse 
phase)  dispersed  through  a  continuous  medium  (con- 
tinuous phase)  of  another.  When  the  continuous  phase 
of  a  colloid  is  a  liquid  such  as  water  the  colloid  is 
called  a  sol,  and  certain  sols  as  the  result  of  the  loss 
of  water,  or  of  other  causes,  pass  into  a  jelly-like 
conditon  known  as  a  gel,  in  which  the  disperse  particles 
are  more  aggregated,  though  there  is  no  sharp  limit 
between  the  sol  and  the  gel  conditions. 

Protoplasm,  as  we  saw,  is  a  mixed  colloid  sol  or 

389 


39°  CONCLUSION 

gel,  often  fluctuating  between  the  two  conditions,  with 
particles  of  protein,  or  loose  combinations  of  proteins 
and  fats  or  proteins  and  salts  as  the  disperse  phase, 
and  water,  with  various  substances  in  solution,  as  the 
continuous  phase  ;  and  it  is  on  this  definite  chemical 
nature  and  physical  structure  that  the  definite  forms 
of  activity — the  "  vital  functions  " — which  are  dis- 
played by  all  living  beings,  depend.  So  much  is  certain, 
though  we  are  still  far  from  understanding  in  detail 
exactly  how  all  the  processes  which  take  place  in  the 
protoplasmic  sol  or  gel  result  in  the  characteristic  life 
phenomena.  In  some  cases  we  can  already  partly 
understand  how  this  happens — as  for  instance  in  the 
case  of  the  amoeba  taking  certain  objects  into  its  body 
and  casting  out  others  (p.  69) — and  as  the  study  of 
the  constitution  and  activity  of  protoplasm  progresses 
we  are  steadily  getting  to  understand  more  and  more 
of  these  details. 

Perhaps  the  two  most  fundamental  properties  or 
powers  of  protoplasm  are  first  its  power  of  maintaining 
the  equilibrium  of  its  physical  structure  within  a  certain 
range  of  external  conditions  (temperature,  vapour, 
pressure,  etc.),  and  secondly  its  power  of  assimilation, 
i.e.  of  taking  into  and  making  part  of  its  own  structure 
certain  chemical  substances  that  contain  the  elements 
of  which  protoplasm  is  itself  composed.  The  first  of 
these  properties  is  the  basis  of  the  maintenance  of  an 
organism,  of  its  continued  life  under  varying  conditions, 
provided  these  do  not  pass  beyond  certain  limits  ;  the 
second  is  the  basis  of  growth  and  reproduction,  of  the 
increase  in  size  and  number  of  existing  organisms. 
Neither  of  these  powers  is  in  itself  unique  in  nature. 
There  are  many  cases  in  which  a  portion  of  matter 
retains  its  equilibrium  as  a  system  for  a  longer  or 


CHARACTERS  OF  PROTOPLASM          3QI 

shorter  time,  and  many  others  in  which  a  portion  of 
matter  may  be  said  to  assimilate  and  grow,  for  instance 
the  simple  case  of  a  crystal  growing  in  a  solution  of 
the  salt  of  which  it  is  composed  by  adding  to  its  own 
structure  molecules  of  the  salt  from  the  solution.  But 
there  is  no  other  case  in  which  so  complicated  and 
delicately  adjusted  a  system  as  a  unit  of  protoplasm 
(a  free  living  cell)  can  do  both  these  things  indefinitely. 
It  is  the  combination  of  complexity  of  structure  and 
instability  of  its  essential  chemical  constituents  with 
the  power  of  maintaining  equilibrium  of  the  system  as 
a  whole  and  of  adding  to  the  system  by  the  assimilation 
of  fresh  material  from  outside  that  makes  protoplasm 
a  unique  substance  in  nature,  and  what  we  call  life 
a  unique  phenomenon. 

To  these  two  powers — of  self-maintenance  and  of 
assimilation  and  growth — we  must  add  the  power  of 
respiration,  the  power,  that  is,  of  oxidising  organic 
substances,  principally  sugars,  within  the  protoplasmic 
complex,  and  thus  of  setting  free  their  potential  energy, 
which  is  available  for  the  production  of  mass  movement 
and  heat. 

Though  protoplasm  has  everywhere  the  same  general 
chemical  composition  and  physical  structure,  it  differs 
in  certain  details  in  every  different  kind  of  animal  and 
plant.  If  we  accept  the  conclusion  that  all  the  pheno- 
mena of  life  depend  upon  the  chemical  and  physical 
composition  and  structure  of  protoplasm,  we  must 
believe  that  the  differences  in  the  manifestations  of 
life  shown  by  different  kinds  of  organisms  depend  on 
differences  of  composition  and  structure,  just  as  the 
essential  general  uniformity  of  life  depends  on  the 
essential  general  uniformity  in  the  composition  and 
structure  of  the  protoplasm  of  all  organisms.  And  this 


3Q2  CONCLUSION 

belief  is  obtaining  more  and  more  detailed  support 
from  recent  research  into  the  biochemistry  of  different 
species  (p.  43). 

The  differences  between  closely  allied  but  distinct 
kinds  of  animals  and  plants  probably  depend  mainly 
on  comparatively  small  differences  in  some  of  the 
proteins  of  which  their  protoplasm  is  composed,  or  on 
differences  in  the  forms  of  aggregation  of  their  protein 
molecules.  When  life  first  appeared  upon  the  earth, 
or  rather  in  the  water,  there  were  probably  different 
kinds  of  very  simple  organisms  having  such  differ- 
ences, which  affected  their  behaviour  and  consequently 
their  mode  of  life.  As  to  the  details  of  such  differences 
we  know  of  course  nothing,  but  it  is  evident  that  they 
must  have  gradually  led  to  the  adoption  of  such  very 
different  modes  of  life  as  we  see  to-day  distinguishing, 
for  instance,  the  animals  from  the  plants.  There  are 
still  existing  many  different  kinds  of  minute  unicellular 
organisms  (Protista)  which  are  neither  unmistakable 
animals  nor  unmistakable  plants,  but  which  show  a 
mixture  of  animal  and  plant  characters.  Yet  the  vast 
majority  of  organisms  are  either  distinctively  animals 
or  distinctively  plants. 

The  main  characteristic  of  animals — on  'which  all 
their  other  features  are  based — is  that  they  can  only 
take  the  nitrogen  which  all  organisms  must  take  in 
to  make  their  proteins  from  ready  made  proteins,  and 
since  proteins  are  only  found  in  nature  in  the  bodies 
or  as  products  of  the  bodies  of  organisms,  animals  must 
consequently  feed  on  other  organisms  or  on  organic 
products.  Plants,  on  the  other  hand,  can  take  their 
nitrogen  in  simpler  forms — as  simple  salts,  or  even  in 
certain  cases  as  free  nitrogen.  The  colourless  plants, 
which  in  the  matter  of  nutrition  are  intermediate 


STRUCTURAL   CHARACTERS   OF   ANIMALS  393 

between  animals  on  the  one  hand  and  green  plants  on 
the  other,  vary  very  much  in  respect  of  the  form  in 
which  they  can  assimilate  nitrogen.  Most  bacteria 
take  it  in  complex  compounds,  but  some  in  simpler  ones, 
while  others  can  fix  free  nitrogen.  The  fungi  proper 
show  a  similar  but  more  restricted  variability.  The 
green  plants,  on  the  whole,  take  their  nitrogen  in  the 
form  of  simple  salts,  such  as  nitrates,  but  even  they 
show  variability.  Some  of  the  lowest  forms  can  take 
their  nitrogen  as  complex  organic  compounds,  and  even 
some  of  the  seed  plants  (which  as  a  class  only  use 
nitrates)  can  feed  on  organic  nitrogenous  compounds. 
This  variability  doubtless  arose  very  early  in  the  history 
of  life,  if  it  did  not  exist  from  the  very  beginning,  and 
was  the  origin  of  the  great  differences  of  structure  and 
mode  of  life  in  existing  organisms. 

The  animals  are  the  most  specialised  class.  Organisms 
which  can  only  take  nitrogen  in  the  form  of  proteins, 
and  thus  from  organic  sources,  were  forced,  so  to  speak, 
into  certain  structures  and  habits,  or  they  could  not 
survive.  They  must  either  have  a  naked  protoplasmic 
surface,  as  amoeba  has,  through  which  solid  food  can 
be  directly  ingested,  or  alternatively  they  must  have  a 
special  opening  in  the  surface  (mouth]  through  which 
food  can  be  taken  in.  A  further  development,  found 
in  the  great  majority  of  animals  is  the  existence  of 
a  cavity  in  the  body  (gut)  into  which  the  mouth  leads 
and  where  the  food  is  digested — broken  up  and  rendered 
soluble — so  that  it  can  be  absorbed  by  the  living  proto- 
plasm ;  and  an  exit  from  this  cavity  (anus)  through 
which  the  indigestible  residue  (faces)  can  be  discharged. 
A  still  further  development  is  a  system  of  tubes 
(circulatory  system,  vascular  system)  which  carries  the 
soluble  products  of  digestion  to  the  living  cells  of 


394  CONCLUSION 

all  parts  of  the  body  where  it  is  actually  assimilated. 
A  further  consequence  of  the  habit  of  living  on  solid 
organic  food  is  that  most  animals — at  least  most  of 
the  higher  animals — have  to  seek  it,  since  it  is  very 
unevenly  distributed  through  their  surroundings,  and 
thus  have  to  move  actively  about,  both  for  this  purpose 
and  in  order  to  escape  from  being  themselves  devoured. 
They  must  move  in  definite  directions  and  capture 
and  devour  their  prey  in  definite  ways.  Therefore 
they  must  be  sensitive  to  a  great  variety  of  external 
impressions  of  definite  kinds,  and  hence  the  gradual 
development  of  the  muscular  system,  the  nervous 
system  and  the  sense  organs. 

The  higher  animals  are,  in  fact,  brought  into  relation 
through  their  specific  life-needs  with  a  much  greater 
range  of  their  environment  than  is  the  case  with  plants, 
and  this  fact  has  called  forth  the  very  high  diff ei  entiation 
and  specialisation  of  their  bodies.  The  necessities  of 
locomotion  have  kept  the  bodies  of  active  animals 
compact,  with  the  different  specialised  organs  and  tissues 
very  closely  dependent  on  one  another,  so  that  no  con- 
siderable part  can  be  detached  without  crippling  or 
killing  the  whole.  This  characteristic  is  known  as  integ- 
ration (the  making  of  a  whole  individual,  the  parts  of 
which  are  closely  interdependent)  and  is  a  feature  of  the 
highest  (vertebrate)  animals  as  well  as  of  such  highly 
developed  specialised  invertebrates  as  the  insects  and 
Crustacea.  Integration  becomes  greater  and  greater  as 
we  ascend  the  animal  scale.  The  lower  animals  are 
much  less  integrated — the  worms  for  instance,  which 
can  be  cut  into  pieces  without  being  killed — and  when 
we  come  down  to  such  forms  as  the  hydroid  polyps  we 
find  that  they  share  many  of  the  characters  of  plants. 
They  are  fixed  branching  forms  of  indefinite  continuous 


EVOLUTION   OF   PLANTS  395 

growth,  and  little  or  no  integration  of  the  body  as  a 
whole,  though  they  are  true  animals,  living  exclusively 
on  organic  food. 

The  powers  of  rapid  locomotion  and  extreme  sensi- 
tiveness to  varied  stimuli,  together  with  the  compactness 
and  high  integration  of  the  body,  are  thus  seen  to  be 
not  necessarily  associated  with  the  fundamental  animal 
character  of  living  on  solid  organic  food,  but  to  be 
merely  a  possible  development  of  animal  organisation. 

The  early  organisms  which  had  the  power  of  living 
on  liquid  foods,  whether  simple  mineral  salts  and 
carbon  dioxide  dissolved  in  water,  or  more  complex 
solutes  such  as  sugars  and  nitrogenous  substances 
derived  from  other  organisms,  were  not  forced  to  retain 
or  develop  the  means  of  ingesting  solid  food,  and  it 
is  these  forms  which  have  given  rise  to  the  class  of 
organisms  we  call  plants.  Those  which  could  intercept 
light  energy  by  means  of  a  pigment  such  as  chlorophyll 
were  able  to  use  the  simplest  forms  of  raw  food  material, 
carbon  dioxide  and  simple  salts,  and  it  is  these  forms 
which  have  given  rise  to  the  green  plants — the  main 
line  of  plant  evolution. 

The  green  plants,  as  the  result  of  their  greenness, 
have  the  unique  power  of  building  up  protoplasm  from 
simple  inorganic  constituents — they  can  assimilate 
carbon  and  nitrogen  from  carbon  dioxide  and  nitrates. 
These  they  find  everywhere  around  them,  the  latter 
in  water  and  soil,  the  former  in  the  air  as  well.  Since 
they  are  able  to  find  their  food  wherever  there  is  enough 
water  to  dissolve  it  and  to  maintain  their  protoplasmic 
structure,  they  are  the  only  possible  forerunners  of 
the  extension  of  life  upon  the  earth,  living  where  other 
organisms  cannot  live,  and  themselves  providing  organic 
food  for  animals  and  for  colourless  plants. 


396  CONCLUSION 

But  the  power  which  green  plants  have  of  construct- 
ing carbohydrates  from  carbon  dioxide  and  water  led, 
broadly  speaking,  to  the  formation  of  more  carbo- 
hydrate material  (sugars)  within  the  cell  than  could 
be  used  for  the  formation  of  new  protoplasm,  since  the 
available  nitrogen  in  the  form  of  nitrates,  though 
widely  distributed,  is  nothing  like  so  abundant  as  the 
carbon  dioxide  and  water.  This  excess  carbohydrate 
material  is  condensed  in  the  form  of  the  polysaccharides 
starch  and  cellulose,  the  latter  covering  the  protoplast 
in  the  form  of  the  characteristic  cell  wall,  which  is  such 
an  important  feature  of  the  structure  of  plants  and 
very  largely  determines  the  configuration  and  structure 
of  their  bodies  and  the  direction  of  their  evolution. 

Under  most  conditions  of  life  green  plants  make  new 
protoplasm  slowly,  and  their  bodies  contain  a  far  smaller 
proportion  of  protoplasm  and  proteins,  and  a  far  larger 
proportion  of  carbohydrates,  than  those  of  animals. 
They  grow  slowly,  continuously,  and  more  or  less 
indefinitely,  and  correspondingly  their  bodies  are  much 
less  highly  integrated  than  the  bodies  of  the  higher 
animals.  An  "  individual  plant  "  is  much  less  of  an 
"  individual  "  than  an  individual  higher  animal,  such 
as  an  insect  or  a  vertebrate.  Large  parts  of  it  can  be 
cut  off  with  little  or  no  injury  to  the  rest  of  the  body, 
and  any  sufficient  portion  (in  some  cases  very  small 
fragments  indeed)  can  grow  into  a  new  plant  under 
suitable  conditions. 

These  features  are  shared  by  the  colourless  plants — 
the  fungi — which  have  not  the  power  of  making  carbo- 
hydrates from  carbon  dioxide  and  water.  But  fungi 
must  live  where  they  can  obtain  at  least  their  carbo- 
hydrate food  ready  made,  and  thus  they  are  not 
pioneers,  like  the  green  plants,  but  camp  followers, 


EFFECT   OF   THE    APPEARANCE    OF   CHLOROPHYLL      397 

like  the  animals.  Their  nutrition  and  metabolism  are 
intermediate  between  the  two,  since  they  can  use 
simpler  nitrogenous  foods  than  animals  can,  and  though 
they  grow  much  more  quickly  than  green  plants  they 
tend,  like  the  latter,  to  form  an  excess  of  carbohydrates. 
Some  authorities  believe  that  the  fungi  are  all  derived 
in  evolution  from  green  plants  through  the  loss  of 
chlorophyll,  but  we  have  no  sufficient  grounds  for 
certainty.  The  fungi  may  have  been  derived,  like  the 
bacteria,  directly  from  primitive  forms  which  had  a 
type  of  nutrition  intermediate  between  that  of  animals 
and  that  of  green  plants. 

We  have  thus  seen  that  the  starting  points  of  the 
great  primary  differentiation  of  living  beings  into 
animals  and  plants  must  have  depended  upon  differences 
in  the  constitution  of  the  protoplasm  of  the  earliest 
forms  of  life,  just  as  the  differences  between  species  in 
existing  organisms  must  ultimately  depend  upon  differ- 
ences between  their  protoplasms.  Some  organisms 
happened  to  produce  the  complex  pigment  chlorophyll 
(p.  112),  which  by  the  absorption  of  light  enabled  the 
protoplasm  to  use  carbon  dioxide  as  food — to  assimilate 
carbon  in  that  simple  form.  Such  organisms  differed 
from  those  which  did  not  produce  chlorophyll  perhaps 
in  no  other  respect,1  and  yet  so  small  a  difference  has 
eventually  led,  in  the  course  of  long  ages,  to  the  differ- 
ence between  a  man  and  an  oak  tree.  And  this  is  the 
story  of  all  evolutionary  development.  The  widest 
differentiations  have  their  origin  in  such  small  differ- 
ences, of  the  order  of  specific  differences,  which  are 
just  sufficient  to  give  the  necessary  bias  to  wholly 

1  We  know  existing  unicellular  organisms  which  differ  from  one 
another  in  just  this  way,  and  consequently,  while  almost  iden- 
tical in  form  and  structure,  have  different  habitats  and  modes  of 
feeding. 


398  CONCLUSION 

divergent  lines  of  development  ;  though  the  converse  is 
by  no  means  true — only  a  few  specific  differences 
actually  lead  to  wide  divergence  in  subsequent  evolution. 
We  are  still  very  ignorant  as  to  the  causes  of  the  first 
appearance  of  such  small  differences.  From  the  point 
of  view  of  their  effects  they  are  chance  occurrences. 

We  have  seen  that  the  activities  of  living  substance 
which  are  common  to  all  organisms — the  vital  functions 
— are  functions  of  the  particular,  the  unique  physico- 
chemical  complex  which  we  call  protoplasm,  and  that 
we  can  partly  explain  how  these  functions  or  activities 
result  from  the  physico-chemical  structure.  We  have 
also  seen  reason  to  believe  that  the  great  primary 
differentiation  between  animals  and  plants  arose  from 
small  differences  in  the  protoplasm  of  different  primitive 
organisms,  and  that  similar  differences  exist  now 
between  closely  allied  species.  There  is  evidence  that 
such  differences  are  still  frequently  occurring  between 
different  individuals  or  groups  of  individuals  of  the 
same  species,  and  that  in  this  way  new  species  originate. 
In  the  higher  forms  of  life,  animals  and  plants  with 
complicated  bodies  of  definite  structure,  such  slight 
changes  in  the  chemical  composition  of  the  protoplasm 
may  be  expressed  in  various  definite  ways,  in  slight 
but  definite  alterations  in  the  form  and  structure,  or 
in  the  colour,  of  the  body  or  of  particular  parts  of  the 
body,  such  as  we  know  is  actually  characteristic  of  the 
differences  between  species.  In  some  cases  such 
changes,  leading  to  the  origin  of  new  species,  can  be 
shown  with  fair  probability  to  be  the  result  of  the 
effect  of  some  change  in  the  surroundings,  for  instance 
increased  dry  ness  or  wetness,  or  the  presence  of  some 
chemical  substance  which  is  absorbed  by  the  body. 
But  in  the  greater  number  they  are  probably  the  result 


SURVIVAL   OR   DISAPPEARANCE   OF   NEW   FORMS      399 

of  changes  in  the  protoplasm  due  to  internal  causes 
which  we  cannot  yet  trace. 

Another  and  probably  very  important  cause  of  the 
appearance  of  new  species  is  the  crossing  of  distinct 
but  closely  allied  species,  the  gametes  contributing  to 
the  zygote  protoplasm  (chromatin)  of  slightly  different 
chemical  composition  or  physico-chemical  structure,  and 
thus  producing  an  individual  of  a  new  type. 

Not  all  new  forms  which  come  into  existence  in  one 
of  these  ways  succeed  in  surviving  and  maintaining 
themselves.  Some  of  the  changes  that  may  occur 
may  make  the  protoplasmic  "machine"  unworkable,  and 
in  that  case  the  new  organism  dies  and  no  new  species 
appears.  In  other  cases  the  change  may  lead  to  a 
modification  of  structure  or  function  which  is  badly 
out  of  harmony  with  the  conditions  in  which  the  new 
organism  finds  itself.  For  instance,  to  take  a  purely 
hypothetical  case,  a  seed  plant  living  in  a  climate  where 
it  was  occasionally  exposed  to  very  dry  air  might 
undergo  a  protoplasmic  change  which  led  to  the  cells 
of  the  epidermis  being  unable  to  form  a  good  cuticle, 
so  that  the  plant  would  rapidly  lose  water  by  evapora- 
tion from  its  epidermal  cells.  Such  a  plant  would  dry 
up  and  die,  and  the  new  form  would  never  establish 
itself. 

But  in  some  cases  the  change  may  actually  lead  to 
the  new  form  being  belter  adapted  to  the  life  conditions, 
and  in  that  case  the  new  form  will  not  only  survive, 
but  will  have  an  advantage  in  the  struggle  for  existence. 
We  saw  in  the  last  chapter  that  a  very  small  proportion 
of  the  seeds  produced  by  a  seed  plant  actually  find 
suitable  conditions  for  germination,  and  that  a  very 
small  proportion  of  the  seedlings  produced  by  the 
seeds  which  do  germinate  grow  into  adult  plants  which 


400  CONCLUSION 

themselves  set  seed.  It  is  clear  that  those  which  are 
best  equipped  for  succeeding  under  the  existing  con- 
ditions will  be  most  likely  to  survive  in  the  struggle 
to  establish  themselves.  For  instance,  suppose  the 
place  where  the  seeds  fall  to  the  ground  is  an  open 
spot  where  heavy  rain  falls  at  intervals  of  several 
weeks,  but  that  there  are  very  dry  periods  between  : 
suppose  further  that  the  surface  layers  of  soil,  say  to 
a  depth  of  2  or  3  inches,  are  very  permeable  to 
water  and  easily  dry  by  evaporation,  but  that  the 
deeper  layers  are  permanently  moist.  If  the  radicles 
of  seeds  germinating  during  the  rain  period  grow  slowly, 
so  that  they  have  not  penetrated  more  than  an  inch 
when  the  dry  period  begins,  the  root  will  be  starved  of 
water  and  the  seedling  wi^l  die.  But  if  a  change  occurs 
in  the  constitution  of  the  seedling  so  that  the  radicle 
grows  quicker  and  the  root  gets  down  to  the  perma- 
nently moist  layers  of  soil  before  the  dry  period  sets 
in,  the  seedlings  will  establish  themselves  and  grow 
into  adult  plants. 

To  take  another  case :  suppose  the  seeds  to  be 
those  of  a  plant  growing  on  the  floor  of  a  forest  where 
the  soil  is  at  all  times  moist,  but  the  light  penetrating 
the  foliage  of  the  trees  is  not  more  than  sufficient  for 
the  needs  of  the  seedlings.  Suppose  the  seeds  of 
various  species  fall  on  the  soil  in  great  numbers, 
and,  the  conditions  for  germination  being  favourable, 
nearly  all  germinate.  Here  there  is  no  difficulty  about 
the  roots — there  is  plenty  of  moisture  in  the  soil  for 
all.  But  the  seedlings  will  grow  up  very  crowded  and 
will  cut  off  the  light  from  one  another.  Those  which 
grow  most  rapidly  will  rise  above  the  others,  will  get 
all  the  available  light,  and  will  establish  themselves; 
the  slower  growing  seedlings  will  be  badly  shaded  and 


NATURAL   SELECTION   AND    ADAPTATION  40! 

will  eventually  die.  Any  change  in  the  constitution 
of  the  seedling  of  a  species  under  these  conditions 
which  enables  its  shoot  to  grow  more  quickly  will  give 
it  a  decisive  advantage  in  the  struggle  for  light. 

In  the  former  case  the  seedlings  were  struggling  with 
their  inorganic  environment,  in  the  latter  competing  with 
other  organisms.  The  world  of  organisms  is  the  scene 
of  both  these  kinds  of  struggle  at  every  stage  of  the 
life  history  of  individual  species,  but  the  struggle  is 
most  severe  in  the  early  stages  of  life,  when  the  indi- 
viduals are  most  numerous  and  have  not  yet  established 
themselves.  This  struggle  for  existence  necessarily  leads 
to  the  success  of  the  forms  which  are  best  equipped  to 
succeed.  There  is  a  selection  of  such  forms  by  the 
elimination  of  those  which  do  not  succeed,  and  it  is 
this  which  Darwin  called  natural  selection. 

One  of  the  first  things  that  must  strike  the  student 
of  biology  is  the  marvellous  adaptation  of  the  different 
kinds  of  plants  and  animals  to  the  conditions  in  which 
they  live,  and  this  adaptation  is  in  large  part  brought 
about  by  the  success  of  those  forms  which  have  changed 
in  directions  advantageous  to  them.  Since  they  have 
succeeded  they  are  the  forms  we  actually  meet  with — 
the  failures  have  disappeared.  But  there  are  many 
degrees  of  success.  Some  species  are.  tremendously 
successful,  spreading  widely  and  constantly  establish- 
ing themselves  in  new  places — carrying,  so  to  speak, 
all  before  them.  Others  hold  their  own,  but  no  more, 
succeeding  perhaps  in  some  places,  failing  in  others. 
Again ,  there  are  species  which  are  not  now  holding  their 
own,  but  are  slowly,  or  rapidly,  dying  out. 

In  the  course  of  evolution  different  species  have 
become  closely  adapted  to  different  kinds  of  life  con- 
ditions— habitats  as  they  are  called — for  instance  hot 
26 


402  CONCLUSION 

moist  climates,  hot  dry  climates,  cold  climates,  and 
so  on ;  and  again  to  seashores,  marshes,  forests,  as 
weeds  of  cultivated  land,  and  to  many  other  special 
habitats.  They  continue  to  grow  in  these  places  and 
no  others,  because  their  economy  is  adjusted  to  these 
particular  conditions  of  life. 

In  all  this  we  see  varied  special  cases  of  the  great 
universal  law  of  equilibrium,  which  governs  all  the 
processes  of  which  we  have  any  knowledge,  from  the 
movements  of  the  planets  to  those  of  molecules,  atoms 
and  electrons,  from  the  activity  of  protoplasm  to  the 
vagaries  of  the  human  mind.  All  things  which  exist 
are  constantly  tending  towards  positions  of  balance  or 
equilibrium,  i.e.  of  relative  repose,  and  all  activity,  all 
motion,  represents  some  phase  of  this  universal  process. 
The  universe  consists  of  the  most  varied  kinds  of 
systems  in  relatively  stable  or  unstable  equilibrium, 
and  every  fresh  disturbance  of  equilibrium  from  out- 
side any  system  leads  to  fresh  activity  in  the  system 
which  tends  towards  the  establishment  of  a  new 
equilibrium. 

A  free-living  unit  of  protoplasm  represents  a  very 
striking  special  case  of  a  system  of  particles  which  can 
maintain  a  moving  equilibrium,  by  assimilation  of  fresh 
particles  of  special  kinds  of  matter  from  without,  and 
the  breaking  down  (katabolism,  respiration)  of  certain 
substances  within.  This  double  process  it  can  only 
maintain  within  a  certain  range  of  external  conditions. 
Outside  these  the  equilibrium  is  destroyed  and  the 
organism  dies,  the  matter  of  which  it  is  composed 
breaking  up  into  simpler  systems  of  dead  matter  which 
return  to  simpler  states  of  equilibrium.  By  assimila- 
ting faster  than  it  katabolises  the  protoplasmic  unit 


EXTERNAL   AND   INTERNAL   EQUILIBRIUM  403 

grows,  till  its  equilibrium  can  no  longer  be  maintained 
as  a  single  system,  and  it  divides,  breaking  up  into  fresh 
units  like  itself.  When  these  units  separate  reproduction 
takes  place,  when  they  remain  together  a  multicellular 
body  arises.  The  different  cells  of  this,  because  they  are 
differently  placed  in  regard  to  each  other  and  to  the 
environment,  are  differently  affected  by  each  other  and 
by  the  environment,  and  differentiation  begins  between 
the  different  cells.  A  new  equilibrium  tends  to  be 
established,  and  we  have  a  characteristic  form  and 
structure  developed.  This  must  be  in  general  harmony 
with  (i.e.  in  equilibrium  with)  the  external  conditions, 
or  the  organism  will  die  and  disappear.  Only  those 
forms  which  are  in  harmony  with  their  surroundings 
will  survive,  and  it  is  only  those  which  we  see  around 
us.  But  the  possible  kinds  of  harmony  are  very  varied, 
for  in  the  first  place  the  protoplasm  of  different  kinds 
of  organism  is  different,  owing  to  the  perpetual  changes 
which  go  on  in  the  protein  complexes  that  form  the 
basis  of  protoplasm,  the  perpetual  readjustments  of 
internal  equilibrium ;  and  in  the  second  place  the 
external  conditions  are  different  for  different  individuals. 
The  constant  interactions  of  one  and  the  other  are  as 
constantly  bringing  about  fresh  adjustments,  new 
positions  of  the  total  equilibrium  of  the  whole  organism 
— in  other  words,  different  species  of  organisms. 

These  are  the  causes  of  the  immense  variety  in  the 
forms  and  structures  of  the  organisms  that  we  see 
around  us,  and  of  the  fact  that  they  are  in  general, 
and  very  often  in  the  minutest  particulars,  wonderfully 
well  adapted  to  their  surroundings.  Otherwise  they 
could  not  come  into  existence.  But  alongside  of, 
coexisting  with,  the  features  of  structure  and  function 
that  are  thus  adapted,  we  see  many  others  which  are 


404  CONCLUSION 

not.  There  is  no  bar  to  the  appearance  of  characters 
which  are  of  no  use  to  the  organism,  nor  even  of  char- 
acters which  are  disadvantageous  to  it,  provided  they 
do  not  handicap  the  organism  sufficiently  to  destroy  its 
chances  of  continued  existence. 

Proof  that  a  structural  or  functional  character  is  of 
use  to  an  organism  is  no  explanation  of  the  origin  of 
the  character  in  question.  A  useful  character,  even 
when  the  organism  could  not  survive  without  it,  is 
merely  an  expression  of  a  partial  equilibrium  with  the 
environment.  The  origin,  that  is  the  causation  of  a 
character,  is  not  to  be  sought  along  these  lines,  but  by 
studying  the  play  of  forces  at  work.  We  have  con- 
sidered several  cases,  for  instance  the  origin  of  the 
sexual  differentiation  of  algal  gametes  (pp.  200-202), 
the  causes  01  the  differentiation  between  the  external 
and  internal  cells  in  the  Brown  Seaweeds  (p.  228),  the 
ability  of  the  fruits  or  seeds  of  many  seashore  and 
riverside  plants  to  float  in  water  for  a  long  time  with- 
out injury  (p.  372),  in  which  it  is  clear  that  the 
causation  of  these  characters  is  distinct  from  their 
advantageousness  to  their  possessors.  Their  usefulness 
may  explain  their  maintenance,  but  not  their  origin. 

All  characters  alike,  essential,  useful,  useless,  or 
harmful,  are  the  inevitable  products  of  an  organism's 
constitution — in  the  last  analysis  of  the  constitution  of 
its  protoplasm — reacting  according  to  the  laws  of 
physics  and  chemistry  to  the  conditions  of  its  environ- 
ment. The  science  of  biology,  like  every  other  science, 
is  concerned  with  causation,  the  fixed  relations  between 
phenomena,  and  it  is  only  by  the  study  of  the 
causation  of  the  characters  of  living  organisms  that 
our  knowledge  of  biology  can  be  advanced. 


INDEX 


achromatic  spindle,  104-106,  278 

adaptation,  401 

adsorption,  53 

adventitious  roots,  261,  288,  235 

aecidiospores,  179-181 

aerial  (subaerial)  shoot,  255-261, 

263,  264,  266 

aerobic  respiration,  132,  135,  150 
aggregation  of  protein  molecules, 

63 

albuminous  seed,  375 

alcoholic  fermentation,  132-136 

anabolism,  79 

anaerobic  respiration,  132, 135, 150 

Algse,   29,   72,   73,   96,    184,   203, 

206,  207,  214 
alternate  hosts,  181 
amino-acids,  122 
Amceba,  64-72,  74,  79,  81,  83,  84, 

92,  96,  393 
andraecium,  345 
Angiosperms,  28 
animals    and    plants,    differences 

between,  22-26,  206,  207,  392- 

397 

annual  plant,  256,  257 
annual  rings,  335,  336 
anther,  346,  347.  35 L  355~357 
antheridium,    223,   224,    234-236, 

244,  245,  250,  251 
antipodal  cells,  349,  350 
apical  cell  (of  Fucus),  219 
apical    (primary)    meristem,    289, 

291,  297,  320,  363,  377 
apocarpous,  352 
archegonium,  235,  236,  244,  245, 

250,  251 

"  aromatic  substances,"  44 
assimilation,  69,  78,  390,  402 
autumn  wood,  335,  336 
axil,  258 
axillary  bud,   258,   259,   262-265, 

322,  323,  326,  330 


bacillus,  139,  148,  149,  154-156 
Bacteria,   30,   51,    113,    127,    136, 

138-156,  393 

bacteriology,  148 

bark,  299,  329,  340,  341 

binary  fission,  72,  88 

bone,  98 

Bordeaux  mixture,  178 

bordered  pits,  276-278 

bracts,  345 

brewing,  132-134 

Brownian  movement,  50,  62 

Bryophytes,  29 

bud  scale  scars,  330,  342 

bud  scales,  329,  330,  342 

buds,  256-259,  261-266,  316,  322, 

223,  326,  328-330,  378,  381 
bulb  scales,  265 
bulbils,  265 
bulbs,  265,  266 

callose,  274 

calyx,  345,  353,  364 

cambium,  298,  325,  328,  331,  336 

carbohydrates,  37-41,  81,  90,  92, 

120,     122,     152,     154,     159,    220, 

274,  278,  285,  375,  377,  380, 

396,  397 

carotin,  112,  186,  214 
carpels,  346,  352.  353-  354.  355. 

365-368,  369 
cartilage,  95,  98 
catalysts,  44 
cell,   31,   32,   54,  56,   58,  66,   So, 

92-109,  391 
cell  doctrine,  96-98 
cell  sap,   101,   107,  109,  in,  1 1 8, 

290,  296 
cell  wall,  31,  72,  73,  98,  99-ioi, 

396 

cellular  structure,  31 
cellulose,  41,  45,  47,  92,  96,  98, 

107,    122,     128,    136,     141,    152, 


4o6 


INDEX 


154,   186,  270,  275,  276,  278- 

281,  284,  377,  380 
cereals,  381 
chemosynthesis,  115 
chemotaxis,  87 
chemotropism,  351 
Chlamydomonas,  24,  87    185-192, 

r94.  X95.  201,  203 
chlamydospores,  162 
chlorophyll,  23,  32,  112-114,  117, 

178,  214,  302,  317,  395,  397 
chloroplasts,   32,   72,   73,   74,  80, 

102,   112,    113,    115-120,    185, 

190,  233,  234,  236,  242,  302, 

303 
chromatin,  66-68,  103,  106,  128, 

129,  140,  399 
chromosomes,  104-106 
cilia,  85,  93,  141 
classification,  26-28 
Clubmosses,  29,  248,  249 
coagulation  of  proteins,  62 
coccus,  139,  148,  154 
coenobium,  192-199 
coffee  disease,  181 
collenchyma,  279,  311,  317,  320 
colloids,  48-60,  61,  62,  141,  389 
companion  cells,  272,  273,  275,  293, 

324.  332 

competition,  384,  401 
conceptacle,  216,  222-225 
conducting  cells  (of  Fucus),  222 
conidia,  164,  173,  176,  177 
connective  tissue,  95,  98 
continuous  phase,  49,  61,  390 
contractile  vacuole,  65,  84,  186 
cork,  284,  299,  331,  337-342 
corms,  263,  264,  265,  266,  325 
corolla,  345 
cortex,  291-293,  296,  297,  301 

317,  320,  321,  322,  326 
cortex  (of  Fucus),  217,  220,  222 
cotyledons,    256,    257,    362,    363, 

378-380 

crystalloids,  49,  51,  55 
cuticle,   115,  242,  284,   303,   307, 

308 

cutin,  284,  293,  294,  297,  303,  305 
cytase,  45,  272,  377,  380 
cytoplasm,  66,  67,  73,  74,  93,  96- 
98,  100,  101,  107,  108,  112,  115, 
127,    158,   161,   164,   185,   186, 
189,    190,   200,   202,   207-209, 
217,  218,   223,   236,   270,   274, 
276,  278,  286,  348,  350,  376 


"  damping-off,"  172-3 

death  rate  of  seedlings,  383 

dialysis,  55 

diastase,  45,  120,  272,  377,  380 

differentiation,  80,  93,  94,  215,  238 

differentiation,    intracellular,    81, 

93 
diffusion,  54,  55,  99,  114,  116,  118, 

222,  339 

disease  bacteria,  154-156 
disperse  phase,  49,  55,  61,  62,  390 
dormant  buds,  330 

ectoplasm,  62,  101,  102 
egg  apparatus,  350 
elaters,  235,  237,  240,  279 
embryo,  237,  240,  244,  245,  251, 

255.  288,  361,  362,  363,  364, 

374-377-  386-388 
embryonal  cell,  361,  362 
embryonic  cells,  100-102,  270,  320 
emulsoid,  49 
endodermis,   291,   293,   294,   296, 

297.  317-319 
endoplasm,  62,  102 
endosperm,    349,    352,    361,    362, 

364.  374-380,  388 
endospermic  seed,  375,  379,  380 
energid,  97,  98 
energy,  25,  26,  38,  42,  77,  81,  82, 

84,  85,  121,  130,  132,  142,  381 
enzymes,  44-46,  67,  94,  120,  132, 

150,  272,  274,  377 
epicotyl,  258,  362,  378-380 
epidermis,  115,  242,  272,  273,  275, 

278,   284.    302,    303,    305-307, 

309,  310,  316,  321,  338,  339 
epigeal  cotyledons,  378,  379,  387, 

388 

epigynous,  353,  354 
equilibrium,  55,  57,  108,  404 
equilibrium,  law  of,  55,  402,  403 
ethyl  alcohol,  132 
etiolation,  316 
Eudorina,  194,  195 
exalbuminous  seed,  375 
eyespot,  186 

facultative  gametes,  204 

facultative  parasites,  158,  169 

families  of  plants,  27 

fat-cells,  95 

fats,  41,  42,  61,   63,   77,  91,  95, 

120,   154,  376,  377,  381,  382, 

390,  392,  396 


INDEX 


407 


feeding,  68,  77,  78 

female  gamete,  191,  195,  200,  202, 

210.  344,  350-352 
fermentation,   130,   132-136,    138, 

150,  151 

Ferns.  29,  244-248 
fibres,  218,  279,  280,  311 
filament,  346 

filamentous  green  algae,  203 
flagella,  85,  141,  172,  186,  192,  193, 

198-200,  235,  236,  244.  245 
floral  leaves,  258,  344-346,  352- 

flowX?  258,  344-360 
flower  bud,  257,  258 
Flowering  Plants,  28 
foliage  leaf,  22,  242,  244,  246,  256, 

302-314,  315.  325 
food  vacuoles,  69 
foot,  237,  244,  245 
formaldehyde,  114 
frond,  215,  216,  220 
fruit,  258,  361-374 
fruit  buds,  329 
fucoxanthin,  214 
Fucus,  214-230,  240,  351 
Fungi,  30,  96,127, 157-i83,  231,393 

gametes,  188-192,  195,  198,  204, 
205,  209,  222,  234,  239,  250, 
252.  344,  349,  350,  352 

gelatine,  54,  59,  285 

gel  membrane,  54-56,  58,  62,  66, 
101,  103 

gels,  53,  54,  58,  59,  62,  63,  66,  103, 
285 

genera,  27,  139,  148 

geotropism,  87,  288,  315 

germ  cells,  185,  197,  198,  206,  207, 
267 

gland  cells,  94,  95,  272 

glucose,  38,  39,  81,  114,  120 

glycogen,  26,  129 

growth,  79,  87,  88,  90,  106,  160, 
390 

guard  cells,  242,  303,  305-3O8 

Gymnosperms,  28,  385 

gynaecium,  346,  353,  354 

habitats,  401 
haemoglobin,  43,  82 
haustoria,  300,  372 
heartwood,  337 

herbaceous  plants,  255-258,  267, 
328,  329 


heterogametes,  190 

heterospory,  249-252 

holdfast,  215,  219 

holophytic,  78 

holozoic,  78 

Horsetails,  29,  249 

humus,  125,  152,  166 

hydrotropism,  87,  289 

hyphae,  157,  164 

hypocotyl,  256,  257,  362,  378-380 

hypogeal  cotyledons,  379,  380,  387 

hypogynous,  353,  354 

"  immortality  "  of  unicellular 

organisms,  184,  185 
inferior  ovary,  353,  354,  368 
inflorescence,  329,  345 
integration,  394,  395 
intercellular  spaces,  99,  115,  285, 

293.  304.  309,  3io.  335.  339 
intercellular  substance,  98 
"  internal  atmosphere,"  115,  285 
internodes,  258,  316,  345 
invertase,  45 
irreversible  gel,  62 
isogametes,  190,  204 

karyokinesis,  103-106,  209 
katabolism,  79,  83,  84,  402 
kinetic  energy,  25,  38,  81-83,  85, 

121 

laevulose,  38,  39,  45 
lenticels,  339,  341,  343 
leucoplasts,  119 
Lichens,  30 
lignification,  283,  284 
lignin,  283 
lignocellulose,  283 
Upases,  45,  377 
liver,  26,  83 

Liverworts,  29,  232-238 
Lycopods,  248,  249 

male  gametes,  191,  195,  200,  202, 
210,   250,   252.   344,   347,   350. 

352 

maltase,  45,  120 
malting,  133,  381 
maltose,  38,  45 
mechanical  tissue,  278,  279,  312, 

320 

medulla,  217 
megasporangium,    249-252,    254, 

255.  344-  345.  347-  348,  385 


408  INDEX 


megaspore,  249-252,  344,  348~35O, 

384,  385 
meristem,  100,  102,  258,  270,  289, 

320,  321,  377 
meristem,    secondary,    298,    299, 

328,  330,  331 
meristematic  cells,  101-103,   127, 

320,  321 
mesophyll,     115-117,     242,     270, 

302-305,  307,  309-3H,  313 
metabolism,  62,  65,  122 
metabolites,  62 
metaphloem,  324 
metaxylem,  295,  296,  324 
micropyle,  348,  349,  35°,  352,  378, 

38? 
microsporangium,    249-252,    344, 

345-347 
microspore,    249-252,     344,     346, 


M7,  348,  384,385 


middle    lamella,     107,    276,    278, 

285,  338 

midrib,  215,  233,  234,  310 
Mosses,  29,  232,  238-241 
movement,  22-24,  68,  84-87,  93 
Mucor,  158-164 
multicellular  organism,  66,  80,  93, 

96,  206,  403 

muscle  tissue,  94,  95,  98 
must,  134 
mycelium,  157,  164 

natural  selection,  401 

nectary,  272,  356,  357 

nitrification,  153 

nodes,  258,  316,  345 

nomenclature,  27 

non-cellular  plants,  97,  158 

non-endospermic  seed,  375,  380 

nucellus,  348,  349,  364,  385 

nuclear  membrane,  66,  103,  104, 
106 

nucleolus,  100,  103 

nucleus,  62,  66-68,  73,  95,  97, 
100,  102,  103-106,  107,  117, 
125,  139,  158,  162,  185,  187, 

189,     191,    2OI,     2O7,    217,    2l8, 

223-225,    226,    236,    272-274, 

347-350. 352, 361 

obligate  parasite,  157 

oils,  42,  376,  381,  382 

oogonium,  223 

"  organic  "  compounds,  36,  38 

osmosis,  57,  311 


osmotic  pressure,  57,  108,  131,  305 
osmotic  substances,  57,  108 
ovary,   348,   35  L   353,   354,   365- 

368 
ovule,  254,  255,  348,  349,  359,  360, 

361 

palisade   layer    (of    Fucus),    217, 

222 

Pandorina,  192-194 
pappus,  370,  374,  383 
parasites,  78,   154,   158,   169-183, 

300 

parasitic,  23,  78,  157,  300 
parenchyma,  270,  271 
parthenogenesis,  172 
pectin,  285 
pedicel,  345 
peduncle,  345 
Pellia,  232-238 
Pellionia,  126,  234 
Penicillium,  164-166 
pentosans,  275,  285 
perennation,  265 
perennial,  256 
perianth,  345 

pericarp,  364,  365,  368,  369-371 
pericycle,  293,  317,  318,  320,  338, 

34° 

perigynous,  353.  354-  3°° 
perisperm,  364,  387 
petals,  345,  346,  353,  355 
phaeoplasts,    214,   217,    218,    223, 

225,  228 

phellogen,  299,  331,  337-341 
phloem,   292,  293,  295,   297-299, 

310,  317,  318 

phloem  fibres,  332,  334,  336,  337 
photolysis,  115,  285 
photosynthesis,  32,  113-115,  118, 

121,   220,   302,   303,   309 

phototaxis,  87 

phototropism,  87,  160,  289,  315 
Phytophthora,  175-177 
piliferous  layer,  290,  292,  297 
pith,  295,  317,  319,  321 
pits,  108,  276-278,  279-282 
placenta,  348,  349,  355,  368 
placentation,  355,  368 
plankton,  184 
plant  pathology,  181 
plasmolysis,  109,  in,  213 
alastids,  102,  108,  119,  127,  270 
Pleodornia,  195-197,  206 
pollen,  347,  355~358 


INDEX 


409 


pollen  grains,  346-348,  35 1.  355- 

358.  385 

pollen  sac,  346,  347,  351 
pollen  tube,   347,   349,   350,   351, 

385 

pollination,  351,  354 
pollination,  cross,  351,  355,  358 
pollination,  self,  351,  358 
polysaccharides,  40,  41,  49.  396 
Potato  blight,  175-177 
potential  energy,  25,  38,  42,  81, 

85,  121 

principal  rays,  335,  336 
protandrous,  356,  357 
proteans,  45,  272";  377 
protein  cells,  272,  274 
proteins,  42-44,  49.  61-63.  73.  77- 

122,   130,   142,   151,    154,  272, 

278,  375.  377.  38i.  390.  392 
prothallus,  244,  245,  248-251 
Protista,  30,  392 
Protococcus,  72-74,  80,  92,  93 
protophloem,  324,  332 
protoplasm,  31,  32,  33,  36,  56,  58, 

61-64,   64-70,   76,   77,   82,   84, 

100-101,  389,  404 
protonema,  240 
protoxylem,   295,  296,   297,   322- 

325 

pseudopodium,  62,  65,  68,  71 
Pteridophytes,  29,  243-251,  384 
Puccinia,  178-181 
pulses,  381 

pure  culture,  147,  148 
"  pure  strains,"  67,  149 
putrefaction,  151 
pyrenoid,   72,   73,   185,    186,    190, 

207,  208,  209 
Pythium,  172-175 

rays,  295 

receptacle,  345,  353,  354-  365-368 
reproduction,  71,  73,  87,  184 
respiration,   39,  81-83,    118,   121, 

132.   135.   136,   150,   159.  289, 

305.  376.  377.  39i 
reversible  gel,  54,  62 
rhizoids,  221,  225,  233,  238,  239, 

241,  244,  245 
rhizomes,  257,  259-262,  265,  266, 

38i 

ringworm,  169 
root  cap,  285,  289,  290-292 
root  hairs,  241,  289,  290,  294,  296 
root,  the,  241,  288-301 


runners,  261 

Rust  of  wheat,  178-181 

Saccharomyces,  128,  136 

salts,  73,  77,  123,  130,  142,  152, 

159,   186,   217,  220,  240,  241, 

290,   296,   300,   304,   311,   392, 

393,  395 

Saprolegnia,  171,  172 
saprophytes,  127,  157-168 
saprophytic,  78,  157,  158 
sapwood,  337 
scale  leaves,  259,  262,  325 
secondary  meristem,  298,  328,  331 
secondary  rays,  334-336 
secondary    thickening,    255,    298, 

328 

secondary  tissue,  298,  329 
secretory  cells,  272 
seed,    254,     255,    258,     361-364, 

365,  368,  369-372,  375-388 
Seed  Plants,  28,  243,  252,  254,  255, 

344,  384-386 
semipermeable  membrane,  56,  62, 

108,  109 

sepals,  345,  346,  353 
sexual   differentiation,    184,    190, 

200,205,210  211,234 
sieve  plate,  274 
sieve  tubes,   229,   274,  293,   297, 

311,  324,  332,  333.  337 
sol,  48-50,  53.  54-56,  61-63,  loi, 

389 
solutes,  48,  55-57,  61,   108,  293, 

296,  299 

solution,  colloid,  48 
solution,  crystalloid,  48,  55,  59,  391 
solution,  "  true,"  48,  59 
solvents,  48,  55,  56 
soma,  184,  185,  195,  198,  205,  206 
species,  27,  42,  43,  67,   148,   149, 

398.  401,  403 

spectrum  (of  chlorophyll),  113 
spermophytes,  see  seed  plants 
sperms  (=  male  gametes)  198, 199- 

2OI,     212,    223-225,     230,     235, 
236,    244,    245,  248,    250,    251 

spirillum,  139,  148 

spirochaete,  148,  154,  155 

sporangiophore,  160 

sporangium,  160,  161 

spore  capsule,  237,  240 

spores,    131,    143-145,    146,    151, 

155,  160-162 
sporidia,  179,  180 


4IO  INDEX 


sporogonium,  237,  240 
spring  wood,  335,  336 
stamens,  345,  346.  352,  353.  3^4 
starch,  40,  43,  45,  77,  90,  118-120, 

122,   178,   186,  209,   376,   377, 

381,  39.6 
starch   grains,   40,    46,    117,    118, 

126,  234,  301,  319,  375,  380 
sterilisation,  146,  147 
stigma,  348,  35 L  355,   356,  357, 

364-  385 
stimulus,  26,  86 
stipe,  215,  216,  219 
stolons,  261 
stoma,    115,    242,    303,    305-308, 

317.  339 
stone  cells,  280 
storage  tissue  (of  Fucus),  222 
struggle  for  existence,  401 
style,  346,  348,  351,  364,  37° 
subaerial  shoot,  see  aerial 
suberin,  284,  299,  339 
subordinate  rays,  335,  336 
sucrose,  38,  39,  45 
sugars,  38,  39,  45.  5^,  57.  77,  81- 

83,  114,  117-122,  149,  159,  305, 

310,  380,  395.  396 
supporting  tissue,  279,  320 
surface  energy,  52 
surface  tension,  52,  69,  70 
suspensoid,  49,  59 
suspensor,  361-364 
synergidae,  349,  350 

taproot,  256,  257,  267,  288 
taxis,  87 

teleutospores,  179-181 
terminal  bud,  256,  258,  261,  262, 

265,  316,  320,  321,  328 
testa,  364,  369,  371,  378-380 
thallus,  214,  233 
tissue  mother  cell,  331-334 
tissues,  80,  94,  95,  98,   106,   107, 

217,  238,  241,  242,  270 
tracheids,  280-283 
tracheids,  annular,  281,  295 
tracheids,  pitted,  282 
tracheids,  reticulate,  282 
tracheids,  scalariform,  282 
tracheids,  spiral,  280-282,  295 
transpiration,  304,  310 
tropisms,  87,  288,  315 
tubers,  119,  262,  265,  266,  325 
turgor  (turgidity),   108,   109,  no, 

305.  3°7 


ultra-microscope,  50 
ultra-microscopic  particles,  50,  5 1 , 

61 
unicellular  organism,   66,   93,   96, 

184,  185 
urea,  65,  83 
uredospores,  178-181 
uric  acid,  83 

vacuole,    62,    too,    101,    107-109, 

in,   115,   117,   125,   128,   158, 

207,  217,  218,  270,  273,  274, 

275,  296 

vacuole  wall,  101,  102 
vascular  bundles,  302,   310,   317, 

318,  321,  324,  325 
vascular  cylinder,   291,  293,  294, 

295.   297,   317,   318,   319,   322, 

324.  325 

Vascular  Plants,  241-243 
vascular  system,  242,  246 
vascular  tissue,  317 
vegetative  reproduction,  206,  241, 

248,  266,  267 
veins,  302,  310-312,  325 
vessels,  282,  283,  293,  296,  297, 

324,  325,  332,  335 
vestibule,  306,  308 
"  vital    functions,"    76,    92,    97, 

390 
Volvox,  197-200 

water  cultures,  123 

water,  essential  importance  of,  37 

water  tissue,  275,  276,  310 

wilting,  109,  308,  320 

wind  pollination,  358 

wine-making,  134 

wings,  215,  216,  218 

winter  buds,  329,  330 

woody  plants,  255,  328 

wort,  132-134,  381 

xanthophyll,  112,  214 

xylem,  293,  295,   298,   302,   304, 

310,  311,  317,  318,  331-334 
xylem  (wood)  fibre,  332-335 

yeast   plant,    128-136,    142,    143, 
150 

zoogkea,  141 

zoospores,  172,  173,  175,  176,  203 

zygote,  162-164,  175,  189,  208- 

210,  225,  236,  244,  352,  361 


Printed  in  Great  Britain  by 

UNWIN   BROTHERS,    LIMITED 
LONDON    AND    WOK  I  NO 


